OMED Oncomed Pharmaceuticals, Inc.

ABOUT THIS PROXY STATEMENT/PROSPECTUS

This proxy statement/prospectus, which forms part of a registration statement on Form F-4 filed with the U.S. Securities and Exchange Commission (the “SEC”) by Mereo BioPharma Group plc (“Mereo”), constitutes a prospectus of Mereo under Section 5 of the Securities Act of 1933, as amended (the “Securities Act”), with respect to the ordinary shares, each of £0.003 nominal value, in the share capital of Mereo (the “Mereo Shares” and each a “Mereo Share”), which will be represented by American Depositary Shares of Mereo (the “Mereo ADSs”) to be issued to stockholders of OncoMed Pharmaceuticals, Inc. (“OncoMed”) pursuant to the merger of a wholly-owned indirect subsidiary of Mereo with and into OncoMed, with OncoMed continuing as the surviving corporation in the merger and a wholly-owned indirect subsidiary of Mereo (the “Merger”). This proxy statement/prospectus also constitutes a proxy statement of OncoMed under Section 14(a) of the Securities Exchange Act of 1934, as amended (the “Exchange Act”) and constitutes a notice of meeting with respect to a special meeting of OncoMed stockholders (the “OncoMed Special Meeting”).

No person has been authorized to provide you with information that is different from that which is contained in, or incorporated by reference into, this proxy statement/prospectus. Mereo and OncoMed take no responsibility for, and can provide no assurances as to the reliability of, any other information that others may give you and, if given, such information must not be relied upon as having been authorized. This proxy statement/prospectus is dated March 20, 2019. You should not assume that the information contained in this proxy statement/prospectus is accurate as of any date other than that date. You should not assume that the information incorporated by reference into this proxy statement/prospectus is accurate as of any date other than the date of such information. Neither the mailing of this proxy statement/prospectus to OncoMed stockholders nor the issuance by Mereo of Mereo ADSs in connection with the Merger will create any implication to the contrary.

This proxy statement/prospectus does not constitute an offer to sell, or a solicitation of an offer to buy, any securities, or the solicitation of a proxy, in any jurisdiction to or from any person to whom it is unlawful to make any such offer or solicitation. Information contained in this proxy statement/prospectus regarding Mereo has been provided by Mereo and information contained in this proxy statement/prospectus regarding OncoMed has been provided by OncoMed.

Neither Mereo shareholders nor OncoMed stockholders should construe the contents of this proxy statement/prospectus as legal, tax or financial advice. Mereo shareholders and OncoMed stockholders should consult with their own legal, tax, financial or other professional advisors. All summaries of, and references to, the agreements governing the terms of the transactions described in this proxy statement/prospectus are qualified by the full copies of and complete text of such agreements in the forms attached hereto as annexes.

Neither the SEC nor any state securities commission, nor any securities regulatory authority in any other jurisdiction, has approved or disapproved of the securities to be issued in connection with the Merger or determined if this proxy statement/prospectus is accurate or complete. Any representation to the contrary is a criminal offense. For the avoidance of doubt, this proxy statement/prospectus does not constitute an offer to buy or sell securities or a solicitation of an offer to buy or sell any securities in the United Kingdom or any other state in the European Economic Area or a solicitation of a proxy under the laws of England and Wales, and it is not intended to be, and is not, a prospectus or an offer document for the purposes of the prospectus rules made under Part VI of the United Kingdom Financial Services and Markets Act 2000 (as set out in the Financial Conduct Authority Handbook).

THIS PROXY STATEMENT/PROSPECTUS INCORPORATES ADDITIONAL INFORMATION

This proxy statement/prospectus incorporates important business and financial information about OncoMed from documents that OncoMed has filed with or furnished to the SEC, but that have not been included in this proxy statement/prospectus. Please see “Where You Can Find More Information” and “Incorporation of Certain Documents by Reference” located elsewhere in this proxy statement/prospectus. You can obtain any of the documents filed with or furnished to the SEC by OncoMed at no cost from the SEC’s website at www.sec.gov. You may also request copies of these documents, including documents incorporated by reference into this proxy statement/prospectus (other than certain exhibits or schedules to these documents), at no cost by requesting them in writing or by telephone from OncoMed at the following address and telephone number:

OncoMed Pharmaceuticals, Inc.

Attention: Alicia Hager

800 Chesapeake Drive

Redwood City, California 94063

Telephone number: (650) 995-8200

In addition, if you have questions about the Merger, the OncoMed Special Meeting, or the proposals to be considered at the OncoMed Special Meeting, need additional copies of this document and the annexes to this document or need to obtain proxy cards or other information related to the proxy solicitation, you may contact OncoMed’s proxy solicitor, MacKenzie Partners, Inc., at the following address and telephone number:

1407 Broadway, 27th Floor

New York, New York 10018

Call Collect: (212) 929-5500

or

Call Toll Free: (800) 322-2885

Email: proxy@mackenziepartners.com

In order for OncoMed stockholders to receive timely delivery of the documents in advance of the OncoMed Special Meeting, OncoMed stockholders must request the documents no later than five business days before the date of the OncoMed Special Meeting.

CURRENCIES

In this proxy statement/prospectus, unless otherwise specified or the context otherwise requires:

INDUSTRY AND MARKET DATA

In this proxy statement/prospectus, Mereo relies on and refers to information and statistics regarding market shares in the sectors in which it competes and other industry data. Mereo obtained this information and statistics from third-party sources, such as independent industry publications,

government publications or reports by market research firms, which information Mereo has supplemented where necessary with information from various other third party sources, discussions with Mereo customers and its own internal estimates taking into account publicly available information about other industry participants and Mereo management’s best view as to information that is not publicly available. Mereo believes that these sources and estimates are reliable, but it has not independently verified the information and statistics obtained from them.

TABLE OF CONTENTS

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QUESTIONS AND ANSWERS ABOUT THE MERGER AND THE ONCOMED SPECIAL MEETING

The following questions and answers address briefly some questions you may have regarding the proposed merger and the OncoMed Special Meeting. These questions and answers may not address all questions that may be important to you. Please refer to the more detailed information contained elsewhere in this proxy statement/prospectus, as well as the additional documents referred to in, or incorporated by reference into, this proxy statement/prospectus.

General Questions and Answers about the Merger

What is the proposed transaction on which I am being asked to vote?

You are being asked to vote to approve and adopt the Agreement and Plan of Merger and Reorganization, dated as of December 5, 2018 (as may be further amended from time to time, the “Merger Agreement”), entered into by and among Mereo BioPharma Group plc, a public limited company organized under the laws of England and Wales (“Mereo”), Mereo US Holdings Inc., a Delaware corporation and a wholly-owned subsidiary of Mereo (“HoldCo”), Mereo MergerCo One Inc., a Delaware corporation and a wholly-owned subsidiary of HoldCo (“Merger Sub”), and OncoMed Pharmaceuticals, Inc., a Delaware corporation (“OncoMed”). A copy of the Merger Agreement is included as Annex A to this proxy statement/prospectus. Pursuant to the Merger Agreement, Merger Sub will merge with and into OncoMed, with OncoMed surviving the merger as a wholly-owned subsidiary of HoldCo, and an indirect wholly-owned subsidiary of Mereo (the “Merger”). Following the Merger, OncoMed will no longer be a publicly traded corporation. Mereo and its subsidiaries following the Merger, including OncoMed, are referred to in this proxy statement/prospectus as the “Combined Company.”

Why am I receiving this document and why am I being asked to vote on the Merger Agreement?

OncoMed is holding a special meeting of stockholders, which is referred to in this proxy statement/prospectus as the “OncoMed Special Meeting,” in order to obtain the stockholder approval necessary to approve and adopt the Merger Agreement. Approval and adoption of the Merger Agreement requires the affirmative vote of holders of at least a majority of the outstanding shares of OncoMed common stock entitled to vote thereon. OncoMed stockholders will also be asked to approve, on a non-binding, advisory basis, the compensation payments that will or may be paid by OncoMed or Mereo to OncoMed’s named executive officers and that are based on or otherwise related to the Merger and the agreements and understandings pursuant to which such compensation may be paid or become payable, and to approve the adjournment from time to time of the OncoMed Special Meeting, if necessary or appropriate, to solicit additional proxies if there are not sufficient votes to approve and adopt the Merger Agreement at the time of the OncoMed Special Meeting or any adjournment or postponement thereof, as further described in the section entitled “—What are the proposals on which the OncoMed stockholders are being asked to vote?” elsewhere in this proxy statement/prospectus. It is important that OncoMed’s stockholders vote their shares of OncoMed common stock on each of these matters, regardless of the number of shares owned. The adoption of the Merger Agreement by OncoMed’s stockholders is a condition to the completion of the Merger. See the section entitled “The Merger Agreement—Conditions to Closing” elsewhere in this proxy statement/prospectus.

This document serves as the proxy statement by which OncoMed is soliciting proxies to obtain the necessary approvals from its stockholders for the Merger. It also serves as the prospectus by which Mereo will offer and issue the ordinary shares, with a nominal value of £0.003 per ordinary share, of Mereo (the “Mereo Shares”) underlying the American Depositary Shares (the “Mereo ADSs”) that will be issued to OncoMed stockholders as a part of the Merger Consideration. It provides OncoMed stockholders with important details about Mereo and their rights as potential equityholders of Mereo.

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Is my vote important?

Yes, your vote is very important. For OncoMed stockholders, an abstention from voting or a failure to vote will have the same effect as a vote “AGAINST” the approval and adoption of the Merger Agreement, assuming a quorum is present. If you hold your OncoMed common stock through a broker, bank or other nominee holder of record and you do not give voting instructions to that broker, bank or other nominee holder of record, that broker, bank or other nominee holder of record will not be able to vote your shares on the approval and adoption of the Merger Agreement, and your failure to give those instructions will have the same effect as a vote “AGAINST” the approval and adoption of the Merger Agreement, assuming a quorum is present. OncoMed’s board of directors (the “OncoMed Board”) unanimously recommends that OncoMed stockholders vote “FOR” the approval and adoption of the Merger Agreement.

The enclosed proxy materials allow you to grant a proxy or vote your shares by telephone or Internet without attending the OncoMed Special Meeting. You are encouraged to submit your proxy or vote your shares by telephone or Internet as soon as possible, even if you plan to attend the OncoMed Special Meeting.

What will OncoMed stockholders receive in the Merger?

If the Merger is completed, OncoMed stockholders will receive, in exchange for each outstanding share of OncoMed common stock owned immediately prior to completion of the Merger (except for any dissenting shares): (1) a number of Mereo ADSs determined by reference to the exchange ratio described below (the “Share Consideration”), and (2) one contingent value right (a “CVR”), representing the right to receive contingent payments if specified milestones are achieved within agreed time periods, subject to and in accordance with the terms and conditions of the Contingent Value Rights Agreement, in substantially the form included as Annex B to this proxy statement/prospectus (the “CVR Agreement”), to be entered into at or prior to the effective time of the Merger (the “Effective Time”) by and among Computershare Inc., as rights agent, and Mereo (together with the Share Consideration, the “Merger Consideration”).

Under the exchange ratio formula set forth in the Merger Agreement (the “Exchange Ratio”), as of immediately following the Effective Time, former OncoMed stockholders are expected to own approximately 25% of the outstanding equity interests in the Combined Company on an undiluted basis (the “Implied OncoMed Ownership”), subject to adjustment for the net cash held by OncoMed at the time of the closing of the Merger (the “Closing”). The number of Mereo Shares underlying Mereo ADSs issuable to former OncoMed stockholders in the Merger is also subject to the limitation that the number of Mereo Shares to be allotted and issued by Mereo (and the corresponding number of Mereo ADSs to be issued to holders of OncoMed common stock) as Share Consideration or pursuant to the CVR Agreement will not, in the aggregate, exceed 66.67% of the Mereo Shares issued and outstanding immediately prior to the Effective Time (approximately 40% of the share capital of the Combined Company) (the “Share Consideration Cap”), as described further in “The Merger Agreement—Merger Consideration.” No fractional Mereo ADSs or CVRs will be issued in connection with the Merger. Any fractional Mereo ADSs or CVRs will be rounded down to the nearest whole Mereo ADS or CVR, as applicable, with no cash being paid to compensate for such rounding. Mereo has applied to list the Mereo ADSs on The Nasdaq Stock Market (“Nasdaq”).

Based on the closing price per Mereo Share of £1.90 as of December 4, 2018, which was the last trading day of Mereo Shares on the Alternative Investment Market operated by London Stock Exchange plc (“AIM”) before the announcement of the Merger, the Federal Reserve Bank of New York’s reported U.S. dollar to pound sterling exchange rate on such date of $1.2719 to £1.0000, and assuming no adjustment to the Exchange Ratio for the net cash held by OncoMed at the time of the closing of the Merger, the Share Consideration implied a value of $1.49 per share of OncoMed

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common stock. This represented a premium to OncoMed stockholders of approximately 33.8% over OncoMed’s closing stock price on December 4, 2018.

Based on the closing price per Mereo Share of £1.68 as of March 15, 2019, which was the latest practicable trading day of Mereo Shares on AIM before the publication of this proxy statement/prospectus, the Federal Reserve Bank of New York’s reported U.S. dollar to pound sterling exchange rate on such date of $1.3278 to £1.0000, and assuming no adjustment to the Exchange Ratio for the net cash held by OncoMed at the time of the closing of the Merger , the Share Consideration would imply a value of $1.37 per share of OncoMed common stock.

The Implied OncoMed Ownership referenced above is an estimate only and the actual percentage of outstanding equity interests in the Combined Company to be held by former OncoMed stockholders immediately following the closing of the Merger will be determined by the final Exchange Ratio, calculated pursuant to a formula described in more detail in the Merger Agreement and elsewhere in this proxy statement/prospectus. In particular, if OncoMed’s net cash at the time of the closing of the Merger is less than $36.5 million, the Exchange Ratio will be reduced by an amount that is greater than the amount that would otherwise reflect the difference between the target closing net cash and actual closing net cash on a dollar-for-dollar basis. Because the number of Mereo ADSs to be exchanged for each share of OncoMed common stock will be adjusted based on the net cash held by OncoMed at the time of the closing of the Merger and will be unaffected by any increase or decrease in exchange rates or in the share price of Mereo Shares between now and the closing of the Merger, the notional value of the Merger Consideration and the exact number of Mereo ADSs that will be issued to OncoMed stockholders as of the date of the OncoMed Special Meeting and as of the date of the closing of the Merger cannot be determined with precision in advance of the Effective Time.

What are the CVRs?

The CVRs represent the non-transferable contractual right to receive certain stock and cash payments from Mereo if specified milestones are achieved within agreed time periods. Each share of OncoMed common stock outstanding immediately prior to the Effective Time shall be converted into the right to receive one CVR, in addition to the Share Consideration. A copy of the form of the CVR Agreement is included as Annex B to this proxy statement/prospectus.

The CVR milestones relate to OncoMed’s etigilimab (anti-TIGIT, OMP-313M32) and navicixizumab (anti-DLL4/VEGF, OMP-305B83) therapeutic candidates. The contingent payments become payable to the rights agent, for subsequent distribution to the holders of the CVRs, upon the achievement of the milestones as follows:

The TIGIT Milestone

A payment, in the form of Mereo ADSs, will be made to CVR holders if, following the Effective Time but prior to December 31, 2019, the following milestone (the “TIGIT Milestone”) is achieved:

If the TIGIT Milestone is achieved, holders of CVRs would be entitled to receive a number of Mereo ADSs equal to (x) the amount of the cash payment actually received by OncoMed upon the Celgene Option Exercise, net of any tax and other reasonable expenses, divided by (y) the volume-weighted average price per Mereo ADS for the ten trading day period immediately following the date of

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the announcement by Mereo of the receipt of such cash payment. The TIGIT Milestone payment is subject to the Share Consideration Cap, such that the number of Mereo Shares underlying the Mereo ADSs to be issued pursuant to the CVR Agreement, when aggregated with the number of Mereo Shares underlying the Mereo ADSs issued as Share Consideration pursuant to the Merger Agreement, cannot exceed the Share Consideration Cap.

In the event that the cash payment payable by Celgene pursuant to the Celgene Option Exercise is actually received by OncoMed prior to the Effective Time, OncoMed stockholders will receive value for such cash payment in the form of additional Mereo ADSs issued at the Effective Time pursuant to the application of the Exchange Ratio set forth in the Merger Agreement. In such event, the CVR Agreement will be amended to delete the TIGIT Milestone and all related payment mechanics.

The NAVI Milestones

A cash payment will be made to CVR holders if, within eighteen months following the closing of the Merger, Mereo or any of its subsidiaries enters into a definitive agreement with one or more third parties regarding the navicixizumab products and, within five years of the closing of the Merger, Mereo or any of its subsidiaries actually receives certain eligible cash milestone payments (each, a “NAVI Milestone”).

If a NAVI Milestone is achieved, holders of CVRs would be entitled to receive an amount in cash equal to 70% of the amount of such eligible cash milestone payment, net of any tax and other reasonable expenses. The NAVI milestone payments are subject to a cash consideration cap, pursuant to which the aggregate principal amount of all cash payments made to holders of CVRs by Mereo shall in no case exceed $79.7 million.

The CVRs may not be sold, assigned, transferred, pledged or disposed of in any other manner, in whole or in part, other than in the limited circumstances specified in the CVR Agreement. In addition, the CVRs (i) will not be evidenced by a certificate or other instrument, (ii) will not have any voting or dividend rights and (iii) will not represent any equity or ownership interest in Mereo or any of its subsidiaries or in the surviving corporation. No interest will accrue on any amounts payable in respect of the CVRs.

Mereo’s obligation to make the CVR payment, if any becomes due, is neither secured nor guaranteed. Mereo’s obligation to make the CVR payment, if any becomes due, is an unsecured general obligation of Mereo and is not guaranteed by Mereo or any of its affiliates.

For a more detailed description of the CVRs and the CVR Agreement, see “Description of the CVRs” elsewhere in this proxy statement/prospectus.

After the Merger, how much of the Combined Company will OncoMed stockholders own?

Under the exchange ratio formula set forth in the Merger Agreement, as of immediately following the Effective Time, former OncoMed stockholders are expected to own approximately 25% of the outstanding equity interests in the Combined Company on an undiluted basis, subject to adjustment for the net cash held by OncoMed at the time of the closing of the Merger. Because the number of Mereo ADSs to be exchanged for each share of OncoMed common stock will be adjusted based on the net cash held by OncoMed at the time of the closing of the Merger, the percentage of outstanding Mereo Shares actually held by former OncoMed stockholders after the Merger may be greater or less than 25%. The number of Mereo Shares to be allotted and issued by Mereo (and the corresponding number of Mereo ADSs to be issued to holders of OncoMed common stock) as Merger Consideration or pursuant to the CVR Agreement will not, in the aggregate, exceed 66.67% of the Mereo Shares issued and outstanding immediately prior to the Effective Time (approximately 40% of the share capital of the Combined Company).

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Can the value of the Merger Consideration change between now and the time the Merger is consummated?

Yes, the value of the Merger Consideration can change. The Exchange Ratio that applies to the Share Consideration is subject to adjustment for the net cash held by OncoMed at the time of the closing of the Merger and to the Share Consideration Cap, meaning that former OncoMed stockholders may be entitled to receive Mereo ADSs representing more or less than 25% of the outstanding equity interests in the Combined Company immediately following the Effective Time, regardless of the trading price of Mereo Shares on AIM or currency exchange rates on the effective date of the Merger. The actual market value of the Mereo ADSs that OncoMed stockholders will receive in the Merger may increase or decrease as the trading price of Mereo Shares increases or decreases, or as currency rates fluctuate, and may be different at the time the OncoMed Special Meeting is held or the Merger is consummated than it was as of the last trading day before the Merger Agreement was signed. The market price of Mereo Shares could be higher or lower at any time prior to the consummation of the Merger than the price of Mereo Shares as of the last trading day before the Merger Agreement was signed. Additionally, fluctuations in the currency exchange rate between the U.S. dollar and the pound sterling could affect the value of the Merger Consideration. OncoMed stockholders are urged to obtain current trading prices for Mereo Shares on AIM and current exchange rates between the U.S. dollar and the pound sterling. You should obtain current trading prices of Mereo Shares and OncoMed common stock, and currency exchange rates, before deciding how to vote on the approval and adoption of the Merger Agreement.

What will happen to my OncoMed options or OncoMed restricted stock units in the Merger?

OncoMed Options

At or immediately prior to the Effective Time, each outstanding and unexercised option to acquire OncoMed common stock pursuant to OncoMed’s 2004 Stock Incentive Plan, as amended, or OncoMed’s 2013 Equity Incentive Award Plan, as applicable, whether or not vested (each, an “OncoMed Option”) will be automatically canceled and converted into the right to receive (i) the excess, if any, of the Merger Consideration over the applicable exercise price of such canceled OncoMed Option, multiplied by (ii) the number of shares of OncoMed common stock subject to such OncoMed Option immediately prior to the Effective Time, provided that no fractional Mereo ADSs or CVRs will be issued. Each OncoMed Option that has a per-share exercise price that is higher than the Merger Consideration shall be canceled at the Effective Time for no consideration. No OncoMed Options will remain outstanding following the consummation of the Merger.

OncoMed Restricted Stock Units

Immediately prior to the Effective Time and contingent on the occurrence of the Closing, each outstanding restricted stock unit, representing the right to receive shares of OncoMed common stock in the future pursuant to OncoMed’s 2013 Equity Incentive Award Plan (each, an “OncoMed Unit”) will be canceled and the holders thereof will be entitled to receive, immediately prior to the Effective Time, the number of shares of OncoMed common stock that were subject to such OncoMed Unit (with the tax withholding obligations of each holder of such OncoMed Units being satisfied by OncoMed withholding from issuance that number of shares of OncoMed common stock calculated by multiplying the maximum statutory withholding rate for such holder in connection with such issuance by the number of shares of OncoMed common stock to be issued, rounding up to the nearest whole share, and remitting such withholding in cash to the appropriate tax authority) and such stockholders will thereafter be entitled to receive the Merger Consideration in respect of these shares of OncoMed common stock. No OncoMed Units will remain outstanding following the consummation of the Merger.

What is a Mereo ADS?

A Mereo ADS is an American Depositary Share, which is a security that allows persons in the United States to more easily hold and trade interests in companies incorporated or organized outside

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of the United States. Mereo is a public limited company organized under the laws of England and Wales that issues ordinary shares that are equivalent in many respects to the common stock of a U.S. company. See “Comparison of Shareholder Rights” for a discussion of the differences between OncoMed common stock and Mereo Shares. Each Mereo ADS represents five Mereo Shares. Mereo has applied to list the Mereo ADSs on Nasdaq, under the symbol “MREO.” Citibank, N.A. is the depositary of the Mereo Shares underlying the Mereo ADSs and will be responsible for issuing Mereo ADSs to OncoMed stockholders in the Merger.

Will OncoMed stockholders be able to trade the Mereo ADSs that they receive in the transaction?

Yes. Mereo has applied to list the Mereo ADSs on Nasdaq under the symbol “MREO.” Mereo ADSs received in exchange for shares of OncoMed common stock in the transaction will be freely transferable under United States federal securities laws. Mereo ADSs will be listed for trading, and be quoted, in U.S. dollars.

Can I receive Mereo Shares in the Merger instead of Mereo ADSs?

No. However, you may turn in your Mereo ADSs at the depositary’s corporate office or by providing appropriate instructions to your broker. Upon payment of the fees provided in the deposit agreement and any applicable taxes, the depositary will deliver to you the Mereo Shares underlying your Mereo ADSs held on deposit by the custodian.

What are the material U.S. federal income tax considerations of the Merger for me?

The exchange of OncoMed common stock for merger consideration in the Merger is expected to be a taxable transaction for U.S. federal income tax purposes to U.S. Holders (as defined in “Material U.S. Federal Income Tax Considerations”). Please carefully review the information under “Material U.S. Federal Income Tax Considerations” beginning on page 283 of this proxy statement/prospectus for a description of material U.S. federal income tax consequences of the merger to U.S. Holders. The tax consequences to you will depend on your own situation. You are urged to consult your tax advisors as to the specific tax consequences to you of the Merger and your receipt of the Merger Consideration, including the applicability and effect of U.S. federal, state, local and non-U.S. income and other tax laws in light of your particular circumstances.

What are the material U.K. tax considerations of owning Mereo ADSs for me?

You are referred to the section of this proxy statement/prospectus entitled “Material U.K. Tax Considerations” for a summary of the anticipated material U.K. tax considerations of ownership of Mereo ADSs. You are urged to consult with your own tax advisor for a full understanding of the U.K. tax considerations to you of owning Mereo ADSs.

When is the Merger expected to be completed?

Mereo and OncoMed expect to complete the Merger promptly after OncoMed receives, at the OncoMed Special Meeting, an affirmative vote in favor of the approval and adoption of the Merger Agreement by holders of a majority of the shares of OncoMed’s common stock entitled to vote at the OncoMed Special Meeting (the “OncoMed Stockholder Approval”). Mereo and OncoMed currently anticipate that the Merger will occur in the first half of 2019. However, neither Mereo nor OncoMed can predict the exact timing of the completion of the Merger because the Merger is subject to certain other conditions to closing as set forth in the Merger Agreement. See the section entitled “The Merger Agreement—Conditions to Closing” elsewhere in this proxy statement/prospectus.

What is required to complete the Merger?

Each of Mereo’s and OncoMed’s obligation to consummate the Merger is subject to a number of conditions specified in the Merger Agreement, including (i) the approval and adoption of the Merger

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Agreement by OncoMed’s stockholders and, if necessary, Mereo’s shareholders (the “Mereo Shareholder Approval”); (ii) the absence of any temporary restraining order, preliminary or permanent injunction or any other order preventing the consummation of the Merger and any law that makes illegal the consummation of the Merger; (iii) the SEC having declared effective this registration statement on Form F-4 (the “Form F-4”) and the registration statement on Form F-6 relating to the registration under the Securities Act of the issuance of the Mereo ADSs (the “Form F-6”) to be filed with the SEC; (iv) Mereo having obtained all required shareholder approvals in connection with the issuance of Mereo ADSs and the allotment and issuance of the Mereo Shares underlying the Mereo ADSs to be issued in the Merger and the grant of the CVRs to the stockholders of OncoMed pursuant to the Merger Agreement; (v) the approval for listing on Nasdaq, subject to official notice of issuance, of the Mereo ADSs to be issued in the Merger and the approval for admission to trading on AIM of the Mereo Shares underlying the Mereo ADSs to be issued in the Merger pursuant to the Merger Agreement, and the satisfaction of any other requirements of London Stock Exchange plc; (vi) subject to certain materiality exceptions, the accuracy of certain representations and warranties of each of OncoMed and Mereo contained in the Merger Agreement and the compliance by each party with the covenants contained in the Merger Agreement; and (vii) the absence of a material adverse effect with respect to each of OncoMed and Mereo. The parties expect the Merger will be completed in the first half of calendar year 2019. See “The Merger Agreement—Conditions to Closing” located elsewhere in this proxy statement/prospectus.

On June 2, 2016, holders of Mereo Shares granted authority to the board of directors of Mereo (the “Mereo Board”) to issue and allot the Mereo Shares underlying the Mereo ADSs.

What happens if the Merger is not completed?

If the Merger Agreement is not adopted by OncoMed stockholders or if the Merger is not completed for any other reason, OncoMed stockholders will not receive the Merger Consideration in exchange for their shares of OncoMed common stock. Instead, OncoMed will remain an independent public company and OncoMed common stock will continue to be listed and traded on Nasdaq. Under specified circumstances, OncoMed may be required to pay Mereo a termination fee and reimburse Mereo’s transaction expenses, or Mereo may be required to pay OncoMed a termination fee and reimburse OncoMed’s transaction expenses, as described in “The Merger Agreement—Termination Fees” located elsewhere in this proxy statement/prospectus.

If, for any reason, the Merger does not close, the OncoMed Board may elect to, among other things, attempt to complete another strategic transaction similar to the Merger, attempt to sell or otherwise dispose of the various assets of OncoMed or continue to operate the business of OncoMed. If OncoMed decides to dissolve and liquidate its assets, OncoMed would be required to pay all of its debts and contractual obligations, and to set aside certain reserves for potential future claims, and there can be no assurances as to the amount or timing of available cash, if any, left to distribute to OncoMed stockholders after paying the debts and other obligations of OncoMed and setting aside funds for reserves.

What do I need to do?

After you have carefully read and considered the information contained in, or incorporated by reference into, this proxy statement/prospectus, please vote by submitting your proxy card or voting instruction form by following the instructions set forth below under “Questions and Answers about the OncoMed Special Meeting—How do I vote?”

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Questions and Answers about the OncoMed Special Meeting

When and where is the OncoMed Special Meeting?

The OncoMed Special Meeting will be held at 8:00 a.m., local time, on April 17, 2019 at OncoMed’s headquarters located at 800 Chesapeake Drive, Redwood City, California 94063. Check-in will begin at 7:30 a.m., local time. Please allow ample time for the check-in procedures.

How can I attend the OncoMed Special Meeting?

OncoMed stockholders as of the close of business on March 19, 2019, the record date, and those who hold a valid proxy for the special meeting are entitled to notice of, to attend and to vote at the OncoMed Special Meeting. OncoMed stockholders should be prepared to present photo identification for admittance. In addition, names of record holders will be verified against the list of record holders at the close of business on the record date prior to being admitted to the meeting. OncoMed stockholders who are not record holders but who hold shares through a broker or other nominee (i.e., in “street name”) should provide proof of beneficial ownership at the close of business on the record date, such as a letter from their broker reflecting their stock ownership as of the record date, which is March 19, 2019. If OncoMed stockholders do not provide photo identification or comply with the other procedures outlined above upon request, they will not be admitted to the OncoMed Special Meeting.

What matters will OncoMed stockholders vote on at the special meeting?

OncoMed stockholders will vote on the Merger Proposal, the Advisory Vote Proposal and the Adjournment Proposal, if necessary or appropriate, to solicit additional proxies if there are not sufficient votes to approve and adopt the Merger Agreement.

How many votes are needed for the proposals considered by OncoMed stockholders at the OncoMed Special Meeting?

Approval of the Merger Proposal requires the affirmative vote of the holders of at least a majority of the shares of OncoMed common stock outstanding at the close of business on the record date. Approval of Advisory Vote Proposal and the Adjournment Proposal each requires the affirmative vote of a majority of the votes cast affirmatively or negatively by holders of shares of OncoMed common stock present in person or represented by proxy at the OncoMed Special Meeting.

What is the quorum requirement for the OncoMed Special Meeting?

A quorum of OncoMed stockholders will be present if at least a majority in voting power of the stock issued and outstanding and entitled to vote as of the record date is present in person, or by remote communication, if applicable, or represented by proxy at the OncoMed Special Meeting. Your shares will be counted towards such quorum only if you submit a valid proxy (or one is submitted on your behalf by your broker or other nominee) or if you vote in person at the OncoMed Special Meeting. Abstentions and broker non-votes will be counted towards the quorum requirement. If there is no quorum, the chairperson of the special meeting or a majority in voting power of the stockholders entitled to vote at the meeting, present in person, or by remote communication, if applicable, or represented by proxy may adjourn the OncoMed Special Meeting to another time or date. If you do not vote, it will be more difficult for OncoMed to obtain the necessary quorum to approve the proposals to be considered by OncoMed stockholders at the OncoMed Special Meeting.

As an OncoMed stockholder, how can I vote?

Stockholders of record as of the record date may vote in person by attending the OncoMed Special Meeting or by mail by completing, signing and dating a proxy card or, if you hold your shares in “street name,” a voting instruction form. Proxies and voting instruction forms submitted by mail must be received no later than April 16, 2019 at 11:59 p.m. Eastern Time to be voted at the OncoMed Special Meeting.

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Most stockholders can also vote over the Internet or by telephone. The availability of Internet and telephone voting for shares held in “street name” will depend on the voting processes of your broker or other nominee. If Internet and telephone voting are available, OncoMed stockholders can find voting instructions in the materials accompanying this proxy statement/prospectus. The Internet and telephone voting facilities will close at 11:59 p.m., Eastern Time, on April 16, 2019. Please be aware that OncoMed stockholders who vote by telephone or over the Internet may incur costs such as telephone and Internet access charges for which they will be responsible.

The method by which OncoMed stockholders vote will in no way limit the right to vote at the meeting if you later decide to attend in person. If shares are held in “street name,” OncoMed stockholders must obtain a proxy, executed in their favor, from their broker or other nominee, to be able to vote at the meeting.

Failure by an OncoMed stockholder to submit a proxy, or instruct a broker or other nominee to vote, as the case may be, will have the effect of a vote “AGAINST” the Merger Proposal, assuming a quorum is present, but it will have no effect on the Advisory Vote Proposal or the Adjournment Proposal.

All shares entitled to vote and represented by properly completed proxies received prior to the OncoMed Special Meeting and not revoked will be voted at the meeting in accordance with your instructions. If a signed proxy card is returned without indicating how shares should be voted on a matter and the proxy is not revoked, the shares represented by such proxy will be voted as the OncoMed Board unanimously recommends and therefore “FOR” the Merger Proposal, the Advisory Vote Proposal and the Adjournment Proposal.

For a more detailed explanation of the voting procedures, please see the section entitled “The OncoMed Special Meeting—Voting Procedures” beginning on page 107 of this proxy statement/prospectus.

As an OncoMed stockholder, what happens if I do not vote?

Your vote is very important. For OncoMed stockholders, a failure to vote will have the same effect as a vote “AGAINST” the approval and adoption of the Merger Agreement, assuming a quorum is present. If you hold your OncoMed common stock through a broker, bank or other nominee holder of record and you do not give voting instructions to that broker, bank or other nominee holder of record, that broker, bank or other nominee holder of record will not be able to vote your shares on the approval and adoption of the Merger Agreement, and your failure to give those instructions will have the same effect as a vote “AGAINST” the approval and adoption of the Merger Agreement, assuming a quorum is present. Therefore, OncoMed urges OncoMed stockholders to vote. The OncoMed Board unanimously recommends that OncoMed stockholders vote “FOR” the approval and adoption of the Merger Agreement.

As an OncoMed stockholder, may I change my vote after I have submitted a proxy card or voting instruction card?

Yes. OncoMed stockholders may revoke a previously granted proxy or voting instruction at any time prior to the closing of the polls at the special meeting by:

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If your shares are held in a brokerage account or another nominee, you may change your vote by submitting new voting instructions to your broker or other nominee, or, if you have obtained a legal proxy from your broker, trustee or nominee that holds your shares, by attending the OncoMed Special Meeting and voting in person.

Only the last submitted proxy or voting instruction card will be considered. Please submit a proxy or voting instruction card for the OncoMed Special Meeting as soon as possible.

Should OncoMed stock certificates be sent in now?

No. If the Merger is completed, OncoMed stockholders will receive written instructions for sending in any stock certificates they may have.

What do OncoMed stockholders need to do now?

Carefully read and consider the information contained in and incorporated by reference into this proxy statement/prospectus, including its annexes. In order for OncoMed shares to be represented at the OncoMed Special Meeting, OncoMed stockholders can (1) vote through the Internet or by telephone by following the instructions included on their proxy card, (2) indicate on the enclosed proxy card how they would like to vote and return the proxy card in the accompanying pre-addressed postage paid envelope, or (3) attend the OncoMed Special Meeting in person.

Who can answer questions?

OncoMed stockholders with questions about the Merger or the other matters to be voted on at the OncoMed Special Meeting or who desire additional copies of this proxy statement/prospectus or additional proxy cards should contact:

1407 Broadway, 27th Floor

New York, New York 10018

Call Collect: (212) 929-5500

or

Call Toll Free: (800) 322-2885

Email: proxy@mackenziepartners.com

If you need additional copies of this proxy statement/prospectus or voting materials, contact MacKenzie Partners, Inc. as described above or OncoMed Investor Relations at the following address and telephone number:

OncoMed Pharmaceuticals, Inc.

Attention: Investor Relations

800 Chesapeake Drive

Redwood City, California 94063

Telephone number: (650) 995-8200

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