Exhibit 8.1

                   [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.

      In connection with our opinion we have reviewed and are relying upon the
Registration Statement, including the exhibits thereto, the representations
contained in a letter addressed to us from the Company and dated the date
hereof, and such other documents, records and instruments as we have deemed
necessary or appropriate for purposes of this opinion.

      This opinion is based upon current provisions of the Internal Revenue Code
of 1986, as amended, and the legislative history thereto and Treasury
Regulations issued thereunder, current administrative rulings and practices of
the Internal Revenue Service and judicial decisions, all of which are subject to
change or differing interpretation, possibly with retroactive effect, at any
time.

      Based upon and subject to the foregoing, we hereby confirm that the
discussion set forth in the Registration Statement under the heading "Federal
Income Tax Considerations," subject to the qualifications set forth therein, to
the extent it states matters of law, summaries of legal matters, or legal
conclusions with respect thereto under the laws of the United States, is a fair
summary in all material respects.

      We hereby consent to the filing of this opinion as Exhibit 8.1 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
"Federal Income Tax Considerations" 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