Exhibit 5.1

February 27, 2013

Board of Directors

TESARO, Inc.

1000 Winter Street

Suite 3300

Waltham, MA 02451

Ladies and Gentlemen:

We are acting as counsel to TESARO, Inc., a Delaware corporation (the “Company”), in connection with its registration statement on Form S-1, as amended (File No. 333-186753) (the “Registration Statement”),  filed with the Securities and Exchange Commission under the Securities Act of 1933, as amended (the “Act”), and its related registration statement on Form S-1 pursuant to Rule 462(b) under the Act (the “Related Registration Statement”). The Registration Statement and the Related Registration Statement relate to the proposed public offering of up to $97,704,000 of newly issued  shares of the common stock, par value $0.0001 per share (the “Common Stock”) of the Company (the “Shares”),  all of which shares are to be sold by the Company.  This opinion letter is furnished to you at your request to enable you to fulfill the requirements of Item 601(b)(5) of Regulation S-K, 17 C.F.R. § 229.601(b)(5), in connection with the Registration Statement and the Related Registration Statement.

For purposes of this opinion letter, we have examined copies of such agreements, instruments and documents as we have deemed an appropriate basis on which to render the opinions hereinafter expressed.   In our examination of the aforesaid documents, we have assumed the genuineness of all signatures, the legal capacity of all natural persons, the accuracy and completeness of all documents submitted to us, the authenticity of all original documents, and the conformity to authentic original documents of all documents submitted to us as copies (including telecopies).  As to all matters of fact, we have relied on the representations and statements of fact made in the documents so reviewed, and we have not independently established the facts so relied on.  This opinion letter is given, and all statements herein are made, in the context of the foregoing.

This opinion letter is based as to matters of law solely on the Delaware General Corporation Law, as amended.  We express no opinion herein as to any other laws, statutes, ordinances, rules, or regulations.

Based upon, subject to and limited by the foregoing, we are of the opinion that following (i) execution and delivery by the Company of the Underwriting Agreement, (ii) issuance of the Shares pursuant to the terms of the Underwriting Agreement, and

Hogan Lovells US LLP is a limited liability partnership registered in the District of Columbia.  “Hogan Lovells” is an international legal practice that includes Hogan Lovells US LLP and Hogan Lovells International LLP, with offices in:  Alicante   Amsterdam   Baltimore   Beijing   Berlin   Brussels   Caracas   Colorado Springs   Denver   Dubai   Dusseldorf   Frankfurt   Hamburg   Hanoi   Ho Chi Minh City   Hong Kong   Houston   London   Los Angeles   Madrid   Miami   Milan   Moscow   Munich   New York   Northern Virginia   Paris   Philadelphia   Prague   Rome   San Francisco   Shanghai   Silicon Valley   Singapore   Tokyo   Ulaanbaatar   Warsaw   Washington DC   Associated offices: Budapest   Jakarta   Jeddah   Riyadh   Zagreb.  For more information see www.hoganlovells.com

(iii) receipt by the Company of the consideration for the Shares specified in the resolutions of the Board of Directors, the Shares will be validly issued,  fully paid, and nonassessable.

This opinion letter has been prepared for use in connection with the Registration Statement and the Related Registration Statement.  We assume no obligation to advise you of any changes in the foregoing subsequent to the effective date of the Related Registration Statement.

We hereby consent to the filing of this opinion letter as Exhibit 5.1 to the Related Registration Statement and to the reference to this firm under the caption “Legal Matters” in the prospectus constituting a part of the Registration Statement and the Related Registration Statement.  In giving this consent, we do not thereby admit that we are an “expert” within the meaning of the Act.

2