EXHIBIT 99.1

                                CERTIFICATION OF
                           ANNUAL REPORT ON FORM 10-K

      Pursuant to 18 U.S.C. ss. 1350, as adopted pursuant to ss. 906 of the
Sarbanes-Oxley Act of 2002, in connection with the filing of Omnicom Group
Inc.'s Annual Report on Form 10-K for the year ended December 31, 2002, as filed
with the Securities and Exchange Commission on the date hereof (the "Report"),
each of the undersigned officers of Omnicom certifies that, to such officer's
knowledge:

      o     the Report fully complies with the requirements of Section 13(a) or
            15(d) of the Securities Exchange Act of 1934; and

      o     the information contained in the Report fairly presents, in all
            material aspects, the financial condition and results of operations
            of Omnicom as of the dates and for the periods expressed in the
            Report.

Executed as of March 26, 2003.

                                            /s/ JOHN D. WREN
                                   --------------------------------
                                   Name:   John D. Wren
                                   Title:  Chief Executive Officer and President


                                       /s/ RANDALL J. WEISENBURGER
                                   --------------------------------
                                   Name:   Randall J. Weisenburger
                                   Title:  Executive Vice President and
                                           Chief Financial Officer


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