Exhibit 5.1

                   [Letterhead of Jones, Day, Reavis & Pogue]

                                 April 12, 2002

Omnicom Group Inc.
437 Madison Avenue
New York, NY 10022

Ladies and Gentlemen:

      We have acted as counsel for Omnicom Group Inc., a New York corporation
(the "Company"), in connection with its filing with the Securities and Exchange
Commission of a Registration Statement on Form S-3 (the "Registration
Statement"), with respect to the registration for resale under the Securities
Act of 1933, as amended (the "Act) of $900,000,000 aggregate principal amount of
the company's Zero Coupon Zero Yield Convertible Notes due 2032 (the "Notes")
and the shares of Common Stock, par value $0.15 per share (the "Common Stock"),
issuable upon conversion of the Notes. The Notes were issued pursuant to an
indenture, dated March 6, 2002 (the "Indenture"), by and between the Company and
JPMorgan Chase Bank, as trustee.

      In rendering this opinion, we have examined such documents and records,
including an examination of originals or copies certified or otherwise
identified to our satisfaction, and such matters of law as we have deemed
necessary for purposes of this opinion. Based upon the foregoing and subject to
the qualifications and limitations stated herein, we are of the opinion that (i)
the aggregate principal amount of Notes being registered for resale are validly
issued and outstanding obligations of the Company and (ii) the shares of Common
Stock issuable upon conversion of the Notes are duly authorized and to the
extent shares of Common Stock are issued upon conversion of the Notes in
accordance with the terms of the Notes and the Indenture, such shares of Common
Stock will be validly issued, fully paid and non-assessable.

      Our examination of matters of law in connection with the opinions
expressed herein has been limited to, and accordingly our opinions herein are
limited to, the laws of the State of New York. We express no opinion with
respect to the laws of any other jurisdiction.

      We hereby consent to the filing of this opinion as Exhibit 5.1 to the
Registration Statement filed by the Company to register the resale of the Notes
and Common Stock under the Act and to the reference to us under the caption
"Legal Matters" in the prospectus constituting a part of such Registration
Statement. In giving such consent, we do not hereby admit that we are included
in the category of persons whose consent is required under Section 7 of the Act
or the rules and regulations of the Securities and Exchange Commission
promulgated thereunder.

                                       Very truly yours,

                                       /s/ Jones, Day, Reavis & Pogue

                                       Jones, Day, Reavis & Pogue