OMNICOM GROUP INC.

LETTER OF TRANSMITTAL

Offer to Exchange up to $451,426,000 Aggregate Principal Amount of Newly Issued 4.650% Senior Notes due 2028 (CUSIP No.          ) for a Like Principal Amount of Outstanding Restricted 4.650% Senior Notes due 2028 Issued on December 2, 2025 (CUSIP Nos. 681919BH8 and U68191AG7), up to $591,859,000 Aggregate Principal Amount of Newly Issued 4.750% Senior Notes due 2030 (CUSIP No.          ) for a Like Principal Amount of Outstanding Restricted 4.750% Senior Notes due 2030 Issued on December 2, 2025 (CUSIP Nos. 681919BJ4 and U68191AH5), up to $457,358,000 Aggregate Principal Amount of Newly Issued 2.400% Senior Notes due 2031 (CUSIP No.          ) for a Like Principal Amount of Outstanding Restricted 2.400% Senior Notes due 2031 Issued on December 2, 2025 (CUSIP Nos. 681919BK1 and U68191AJ1), up to $278,341,000 Aggregate Principal Amount of Newly Issued 5.375% Senior Notes due 2033 (CUSIP No.          ) for a Like Principal Amount of Outstanding Restricted 5.375% Senior Notes due 2033 Issued on December 2, 2025 (CUSIP Nos. 681919BL9 and U68191AK8), up to $494,331,000 Aggregate Principal Amount of Newly Issued 3.375% Senior Notes due 2041 (CUSIP No.          ) for a Like Principal Amount of Outstanding Restricted 3.375% Senior Notes due 2041 Issued on December 2, 2025 (CUSIP Nos. 681919BM7 and U68191AL6), and up to $491,657,000 Aggregate Principal Amount of Newly Issued 5.400% Senior Notes due 2048 (CUSIP No.          ) for a Like Principal Amount of Outstanding Restricted 5.400% Senior Notes due 2048 Issued on December 2, 2025 (CUSIP Nos. 681919BN5 and U68191AM4).

THE EXCHANGE OFFER WILL EXPIRE AT 5:00 P.M., NEW YORK CITY TIME, ON                , UNLESS EXTENDED (SUCH TIME AND DATE, AS THE SAME MAY BE EXTENDED, THE “EXPIRATION TIME”). ORIGINAL NOTES TENDERED IN THE EXCHANGE OFFER MAY BE WITHDRAWN AT ANY TIME PRIOR TO THE EXPIRATION TIME.

The Prospectus, dated                , 2025 (the “Prospectus”), and this corresponding Letter of Transmittal, relate to the offer of Omnicom Group Inc. (the “Company”) to exchange (the “Exchange Offer”) up to $2,764,972,000 aggregate principal amount of its 4.650% Senior Notes due 2028, 4.750% Senior Notes due 2030, 2.400% Senior Notes due 2031, 5.275% Senior Notes due 2033, 3.375% Senior Notes due 2041 and 5.400% Senior Notes due 2048 (together, the “Exchange Notes”), which will be issued in a transaction registered under the Securities Act of 1933 (the “Securities Act”), for a like principal amount of the Company’s outstanding restricted 4.650% Senior Notes due 2028, 4.750% Senior Notes due 2030, 2.400% Senior Notes due 2031, 5.275% Senior Notes due 2033, 3.375% Senior Notes due 2041 and 5.400% Senior Notes due 2048 (together, the “Original Notes” and, collectively with the Exchange Notes, the “Notes”).

Each series of the Original Notes was issued pursuant to the Fifth Supplemental Indenture, dated as of December 2, 2025 (the “Fifth Supplemental Indenture”), by and among the Company and Deutsche Bank Trust Company Americas, as trustee (the “Trustee”), to the Indenture, dated as of February 21, 2020, by and among the Company and the Trustee (the “Base Indenture” and, together with the Fifth Supplemental Indenture, the “Indenture”).

Capitalized terms used but not defined in this Letter of Transmittal have the meanings assigned to them in the Prospectus. All terms and conditions contained in, or otherwise referred to in, the Prospectus are deemed to be incorporated in, and form a part of, this Letter of Transmittal. Therefore, you are urged to read carefully the Prospectus and the items referred to therein. The terms and conditions contained in the Prospectus, together with the terms and conditions governing this Letter of Transmittal and the instructions herein, are collectively referred to herein as the “terms and conditions.”

No representation is made as to the correctness or accuracy of the CUSIP numbers listed in this Letter of Transmittal or printed on the Notes. Neither the Company, the exchange agent nor the Trustee shall be responsible for the selection or use of the CUSIP numbers. They are provided solely for the convenience of the Holders (as defined below).

The exchange agent for the Exchange Offer is:

D.F. King & Co., Inc.

Questions relating to the procedure for tendering, as well as requests for additional copies of the Prospectus or this Letter of Transmittal, should be directed to the exchange agent addressed as follows:

By Mail or in Person

D.F. King

Attn: Michael Horthman

28 Liberty Street, 53^rd Floor

New York, NY 10005

By Email Transmission (for Eligible Institutions Only)

Email: omnicom@dfking.com

Banks and Brokers Call for Information:

(212) 401-9970

All Others Call Toll Free:
(800) 290-6432

This Letter of Transmittal is to be used by Holders of each series of the Original Notes. Tender of the applicable Original Notes is to be made using the Automated Tender Offer Program (“ATOP”) of The Depository Trust Company (“DTC”) pursuant to the procedures set forth in the Prospectus under the caption “The Exchange Offers—Procedures for Tendering Original Notes.” DTC participants that are accepting the Exchange Offer must transmit their acceptance to DTC, which will verify the acceptance and execute a book-entry delivery to the exchange agent’s DTC account. DTC will then send a computer-generated message known as an “agent’s message” to the exchange agent for its acceptance. For you to validly tender the applicable series of your Original Notes in the Exchange Offer, the exchange agent must receive, prior to the Expiration Time, an agent’s message under the ATOP procedures that confirms that:

By using the ATOP procedures to tender each series of Original Notes, you will not be required to deliver this Letter of Transmittal to the exchange agent. However, you will be bound by its terms and conditions, and you will be deemed to have made the acknowledgments and the representations and warranties it contains, just as if you had signed it. The tender of the applicable series of Original Notes by you pursuant to the procedures set forth in this Letter of Transmittal and the Prospectus will constitute an agreement between you and the Company in accordance with the terms and subject to the conditions set forth in this Letter of Transmittal and the Prospectus. If you have questions or need help, or if you would like additional copies of the Prospectus and this Letter of Transmittal, you should contact the exchange agent at its telephone number or address set forth above.

The applicable series of Exchange Notes will be issued in full exchange for the corresponding series of Original Notes in the Exchange Offer, if consummated, on the Exchange Date and will be delivered in book-entry form.

As used in this Letter of Transmittal, the term “Holder” means any person in whose name the applicable series of Original Notes are held of record by DTC or its nominee and who desires to deliver such notes by book-entry transfer at DTC.

Please read the accompanying instructions carefully.

Ladies and Gentlemen:

According to the terms and conditions of the Exchange Offer, I hereby tender to the Company the principal amount of the series of Original Notes credited by me to the exchange agent’s account at DTC using ATOP. At the time the applicable series of Original Notes are accepted by the Company and exchanged for the same principal amount of the corresponding series of Exchange Notes, I will sell, assign and transfer to the Company all right, title and interest in and to the Original Notes I have tendered. I am aware that the exchange agent also acts as the agent of the Company. Upon agreement to the terms and conditions of this Letter of Transmittal pursuant to an agent’s message, I irrevocably constitute and appoint the exchange agent as my agent and attorney-in-fact for the tendered series of Original Notes with full power of substitution to:

I represent and warrant that I have full power and authority to tender, exchange, assign and transfer the applicable series of Original Notes that I am tendering and to acquire the corresponding series of Exchange Notes issuable upon the exchange of the tendered series of Original Notes. I represent and warrant that the Company will acquire good and unencumbered title to each applicable series of Original Notes tendered, free and clear of all liens, restrictions (other than restrictions on transfer), charges and encumbrances, and that such Original Notes are not and will not be subject to any adverse claim at the time the Company acquires them. I further represent that:

I understand that the Exchange Offer is being made in reliance on interpretations contained in letters issued to third parties by the staff of the SEC. These letters provide that the applicable series of Exchange Notes issued in exchange for the corresponding series of Original Notes in the Exchange Offer may be offered for resale, resold, and otherwise transferred by a Holder of Exchange Notes, unless that person is an “affiliate” of the Company within the meaning of Rule 405 under the Securities Act, without compliance with the registration and prospectus delivery provisions of the Securities Act, provided that the Exchange Notes must be acquired in the ordinary course of the Holder’s business and the Holder must not be engaging in, must not intend to engage in, and must not have any arrangement or understanding with any person to participate in, a distribution of the applicable series of Exchange Notes.

If I am a broker-dealer that will receive the applicable series of Exchange Notes for my own account in exchange for the corresponding series of Original Notes that were acquired as a result of market-making activities or other trading activities, I represent that I will deliver the Prospectus in connection with any resale of any applicable series of Exchange Notes. However, by this acknowledgment and by delivering the Prospectus, I will not be deemed to admit that I am an “underwriter” within the meaning of the Securities Act.

Upon request, I will execute and deliver any additional documents deemed by the exchange agent or the Company to be necessary or desirable to complete the exchange, assignment and transfer of any applicable series of Original Notes I have tendered.

I understand that the Company will be deemed to have accepted validly tendered Original Notes of any series when and if the Company gives written notice of acceptance to the exchange agent and such acceptance will constitute performance in full by the Company of its obligations under the registration rights agreement, except in the limited circumstances defined in such agreement.

If, for any reason, any tendered Original Notes of any series are not accepted for exchange in the Exchange Offer, the unaccepted Original Notes of such series will be returned without expense to the tendering Holder. Such non-exchanged Original Notes of such series will be credited to an account maintained with DTC. These actions will occur as promptly as practicable after the expiration or termination of the Exchange Offer.

All authority granted or agreed to be granted by this Letter of Transmittal will survive my death, bankruptcy or incapacity, and every obligation under this Letter of Transmittal is binding upon my heirs, legal representatives, successors, assigns, executors, administrators and trustees in bankruptcy.

I understand that tenders of any applicable series of Original Notes according to the procedures described in the Prospectus under the heading “The Exchange Offers—Procedures for Tendering Original Notes” and in the instructions included in this Letter of Transmittal constitute a binding agreement between myself and the Company subject to the terms and conditions of the Exchange Offer.

By crediting the applicable series of Original Notes to the exchange agent’s account at DTC using ATOP and by complying with applicable ATOP procedures with respect to the Exchange Offer, the participant in DTC confirms on behalf of itself and the beneficial owners of such applicable series of Original Notes all provisions of this Letter of Transmittal (including all representations and warranties) applicable to it and such beneficial owner.

INSTRUCTIONS FORMING PART OF

THE TERMS AND CONDITIONS OF THE EXCHANGE OFFER

1. BOOK-ENTRY CONFIRMATIONS.  Any confirmation of a book-entry transfer to the exchange agent’s account at DTC of an applicable series of Original Notes tendered by book-entry transfer, as well as an agent’s message, and any other documents required by this Letter of Transmittal, must be received by the exchange agent at its address set forth on the cover page of this Letter of Transmittal prior to the Expiration Time.

THE COMPANY WILL NOT ACCEPT ANY ALTERNATIVE, CONDITIONAL OR CONTINGENT TENDERS. EACH TENDERING HOLDER BY DELIVERY OF AN AGENT’S MESSAGE WAIVES ANY RIGHT TO RECEIVE ANY NOTICE OF THE ACCEPTANCE OF SUCH TENDER.

Delivery of documents to DTC or to the Trustee does not constitute delivery to the exchange agent.

2. VALIDITY OF TENDERS. The Company has the sole right to decide any questions about the validity, form, eligibility, time of receipt, acceptance or withdrawal of tendered Original Notes of any series, and its decision will be final and binding. The Company’s interpretation of the terms and conditions of the Exchange Offer, including the instructions contained in this Letter of Transmittal and in the Prospectus, will be final and binding on all parties.

The Company has the absolute right to reject any or all of the tendered Original Notes of any series if:

The Company may also decide to waive any conditions of the Exchange Offer or any defects or irregularities of tenders of Original Notes of any series and accept such Original Notes for exchange whether or not similar defects or irregularities are waived in the case of other Holders. Any defect or irregularity in the tender of Original Notes of any series that is not waived by the Company must be cured within the period of time set by the Company.

It is your responsibility to identify and cure any defect or irregularity in the tender of your Original Notes. Your tender of Original Notes will not be considered to have been made until any defect or irregularity is cured or waived. Neither the Company, the exchange agent, the Trustee, nor any other person is required to notify you that your tender was defective or irregular, and no one will be liable for any failure to notify you of such a defect or irregularity in your tender of Original Notes. Tenders of the Original Notes of any series will not be deemed made until such defects or irregularities have been cured or waived. Any Original Notes of any series received by the exchange agent that are not properly tendered and as to which the defects or irregularities have not been cured or waived will be returned to the tendering Holder as soon as practicable after the Expiration Time of the Exchange Offer.

3. PARTIAL TENDERS. Tenders of any series of Original Notes pursuant to the Exchange Offer will be accepted only in principal amounts equal to $2,000 and integral multiples of $1,000 in excess thereof. A Holder will be deemed to have tendered the entire aggregate principal amount represented by the applicable series of Original Notes credited to the exchange agent’s account at DTC using ATOP. Any untendered Original Notes of any series and any corresponding series of Exchange Notes issued in exchange for tendered and accepted Original Notes of any series will be credited to accounts at DTC.

4. TRANSFER TAXES. The Company will pay all transfer taxes, if any, applicable to the exchange of the applicable series of Original Notes in the Exchange Offer. However, transfer taxes will be payable by you (or by the tendering Holder if you are delivering this letter on behalf of a tendering Holder) if a transfer tax is imposed for any reason other than the exchange of the applicable series of Original Notes according to the Exchange Offer. If satisfactory evidence of the payment of those taxes or an exemption from payment is not submitted with this Letter of Transmittal or at the time the related agent’s message is delivered, the amount of those transfer taxes will be billed directly to the tendering Holder. Until those transfer taxes are paid, the Company will not be required to deliver any corresponding series of Exchange Notes required to be delivered to, or at the direction of, such tendering Holder.

Except as provided in this Instruction 4, it is not necessary for transfer tax stamps to be attached to the applicable series of Original Notes listed in this Letter of Transmittal.

5. WAIVER OF CONDITIONS. The Company may choose, at any time and for any reason, to waive or, subject to certain requirements, amend or modify certain of the conditions to the Exchange Offer. The conditions applicable to tenders of any series of Original Notes in the Exchange Offer are described in the Prospectus under the heading “The Exchange Offers—Conditions to the Exchange Offers.”

6. WITHDRAWAL RIGHTS. Original Notes of any series tendered pursuant to the Exchange Offer may be withdrawn at any time prior to the Expiration Time. For a withdrawal to be effective, a written letter notice of withdrawal must be received by the exchange agent at its address set forth in this Letter of Transmittal not later than the Expiration Time. Any notice of withdrawal must specify the name and number of the account at DTC to be credited with withdrawn Original Notes and otherwise comply with the ATOP procedures. The exchange agent will return properly withdrawn Original Notes promptly following receipt of notice of withdrawal. Properly withdrawn Original Notes may be retendered by following the procedures described in the Prospectus under the heading “The Exchange Offers—Procedures for Tendering Original Notes” at any time prior to the Expiration Time. All questions as to the validity of notices of withdrawals, including time of receipt, will be determined by the Company, and will be final and binding on all parties. None of the Company, the exchange agent, the Trustee or any other person will be under any duty to give notification of any defects or irregularities in any notice of withdrawal of tenders, or incur any liability for failure to give any such notification.

7. REQUESTS FOR ASSISTANCE OR ADDITIONAL COPIES. If you have questions, need assistance or would like to receive additional copies of the Prospectus or this Letter of Transmittal, you should contact the exchange agent at the address listed on the cover page of this Letter of Transmittal. You may also contact your broker, dealer, commercial bank, trust company or other nominee for assistance concerning the Exchange Offer.

IMPORTANT: BY USING THE ATOP PROCEDURES TO TENDER ORIGINAL NOTES OF ANY SERIES, YOU WILL NOT BE REQUIRED TO DELIVER THIS LETTER OF TRANSMITTAL TO THE EXCHANGE AGENT. HOWEVER, YOU WILL BE BOUND BY ITS TERMS AND CONDITIONS, AND YOU WILL BE DEEMED TO HAVE MADE THE ACKNOWLEDGMENTS AND THE REPRESENTATIONS AND WARRANTIES IT CONTAINS, JUST AS IF YOU HAD SIGNED IT.

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