Exhibit 5.1

	1271 Avenue of the Americas
	New York, New York 10020-1401
	Tel: +1.212.906.1200 Fax: +1.212.751.4864
	www.lw.com
	FIRM / AFFILIATE OFFICES
	Austin	Milan
	Beijing	Munich
	Boston	New York
	Brussels	Orange County
January 17, 2025	Century City	Paris
	Chicago	Riyadh
	Dubai	San Diego
	Düsseldorf	San Francisco
Omnicom Group Inc. 280 Park Avenue New York, NY 10017	Frankfurt	Seoul
	Hamburg	Silicon Valley
	Hong Kong	Singapore
	Houston	Tel Aviv
	London	Tokyo
Re:  Registration Statement on Form S-4	Los Angeles	Washington, D.C.
	Madrid

To the addressee set forth above:

We have acted as special counsel to Omnicom Group Inc., a New York corporation (the “Company”), in connection with the proposed issuance of up to 128,323,445 shares of common stock, $0.15 par value per share of the Company (the “Shares”), to be issued pursuant to that certain Agreement and Plan of Merger, dated December 8, 2024 (the “Merger Agreement”), by and among the Company, EXT Subsidiary Inc., a Delaware corporation and a direct wholly owned subsidiary of the Company, and The Interpublic Group of Companies, Inc., a Delaware corporation. The Shares are included in a registration statement on Form S-4 under the Securities Act of 1933, as amended (the “Act”), filed with the Securities and Exchange Commission (the “Commission”) on January 17, 2025 (the “Registration Statement”). The term “Shares” shall include any additional shares of common stock registered by the Company pursuant to Rule 462(b) under the Act in connection with the offering contemplated by the Registration Statement. This opinion is being furnished in connection with the requirements of Item 601(b)(5) of Regulation S-K under the Act, and no opinion is expressed herein as to any matter pertaining to the contents of the Registration Statement or related joint proxy statement/prospectus contained therein, other than as expressly stated herein with respect to the issue of the Shares.

As such counsel, we have examined such matters of fact and questions of law as we have considered appropriate for purposes of this letter. With your consent, we have relied upon certificates and other assurances of officers of the Company and others as to factual matters without having independently verified such factual matters. We are opining herein as to the Business Corporation Law of the State of New York (the “NYBCL”), and we express no opinion with respect to any other laws.

Subject to the foregoing and the other matters set forth herein, it is our opinion that, as of the date hereof, when the transactions contemplated by the Merger Agreement have been completed and the Shares have been duly registered in book-entry form on the books of the transfer agent and registrar in the name or on behalf of the recipient, in each case in accordance with the Merger Agreement, the Shares will have been duly authorized by all necessary corporate action of the Company, and the Shares will be validly issued, fully paid and nonassessable.

January 17, 2025

Page 2

In rendering the foregoing opinion, we have assumed that (i) at or prior to the time of the delivery of any Shares, the Registration Statement will have been declared effective under the Act and that the registration will apply to all of the Shares and will not have been modified or rescinded and that there will not have occurred any change in law affecting the validity of the issuance of such Shares and (ii) the Company will comply with all applicable notice requirements regarding uncertificated shares provided in the NYBCL.

This opinion is for your benefit in connection with the Registration Statement and may be relied upon by you and by persons entitled to rely upon it pursuant to the applicable provisions of the Act. We consent to your filing this opinion as an exhibit to the Registration Statement and to the reference to our firm in the joint proxy statement/prospectus contained in the Registration Statement under the heading “Validity of Common Stock.” We further consent to the incorporation by reference of this letter and consent into any registration statement or post-effective amendment to the Registration Statement filed pursuant to Rule 462(b) with respect to the Shares. In giving such consent, we do not thereby admit that we are in the category of persons whose consent is required under Section 7 of the Act or the rules and regulations of the Commission thereunder.

Sincerely,

/s/ Latham & Watkins LLP