Exhibit 99.1

UNAUDITED PRO FORMA CONDENSED COMBINED FINANCIAL STATEMENTS

On December 8, 2024, Omnicom Group Inc. (“Omnicom”) entered into an Agreement and Plan of Merger (the “Merger Agreement”), by and among Omnicom, EXT Subsidiary Inc., a direct wholly owned subsidiary of Omnicom (“Merger Sub”), and The Interpublic Group of Companies, Inc. (“IPG”), pursuant to which, subject to the terms and conditions of the Merger Agreement, Merger Sub will merge with and into IPG (the “Merger”), with IPG surviving the Merger as a wholly owned subsidiary of Omnicom. Each share of IPG common stock will be converted into, and represent the right to receive, 0.344 shares of common stock of Omnicom, plus cash in lieu of any fractional shares of Omnicom common stock that otherwise would have been issued. On March 18, 2025, the shareholders of each of Omnicom and IPG approved the Merger. The completion of the Merger is subject to customary closing conditions, including required regulatory approvals. We have secured regulatory approvals for Omnicom’s pending acquisition of IPG in all required jurisdictions and expect to close the Merger by the close of business on November 26, 2025. The Merger is not subject to a financing condition or the completion of the Exchange Offers or Consent Solicitations announced by Omnicom and IPG on August 11, 2025 related to IPG’s outstanding senior notes (the “Exchange Offers and Consent Solicitations”). Omnicom has received sufficient tenders and consents to consummate the Exchange Offers and Consent Solicitations, which are currently set to expire on November 28, 2025.

The following unaudited pro forma condensed combined financial statements have been prepared in accordance with Article 11 of Regulation S-X and should be read in conjunction with the accompanying notes. The unaudited pro forma condensed combined financial statements have been prepared from the respective historical consolidated financial statements of Omnicom and IPG and reflect transaction accounting adjustments related to the Merger. The unaudited pro forma condensed combined statements of income for the year ended December 31, 2024 and for the nine months ended September 30, 2025 are presented as if the Merger had been completed on January 1, 2024. The unaudited pro forma condensed combined balance sheet as of September 30, 2025 is presented as if the Merger had been completed on September 30, 2025.

The unaudited pro forma condensed combined financial statements have been developed from and should be read in conjunction with:

The unaudited pro forma condensed combined financial statements have been prepared to reflect adjustments to Omnicom’s historical consolidated financial information representing Omnicom management’s estimates based on information available as of November 26, 2025 and are subject to change as additional information becomes available and analyses are performed. However, Omnicom management believes that the assumptions provide a reasonable basis for presenting the significant effects of the Merger as contemplated, and that the pro forma adjustments give appropriate effect to those assumptions and are properly applied in the unaudited pro forma condensed combined financial statements. Accordingly, the unaudited pro forma condensed combined financial statements reflect the following adjustments, which include transaction accounting adjustments:

The unaudited pro forma condensed combined financial statements have been prepared using the acquisition method of accounting in accordance with U.S. generally accepted accounting principles. Omnicom has been treated as the acquirer for accounting purposes, primarily due to Omnicom stockholders holding majority voting interests in the combined company, Omnicom issuing equity interests to effect the Merger and the management of Omnicom continuing to manage the combined entity. Thus, Omnicom will account for the Merger as a business combination in accordance with Financial Accounting Standards Board (“FASB”) Accounting Standards Codification (“ASC”) 805, Business Combinations. The total purchase price will be allocated to the acquired tangible and intangible assets and assumed liabilities based on their respective fair values. The fair values of the acquired assets and assumed liabilities of IPG have been measured based on various preliminary estimates using assumptions that Omnicom management believes are reasonable and based on currently available information. Accordingly, the pro forma adjustments are preliminary and have been made solely for the purpose of preparing the unaudited pro forma condensed combined financial statements. The final purchase accounting could be materially different from the preliminary estimates used to prepare the accompanying unaudited pro forma condensed combined financial statements which could have a material impact on the combined company’s future results of operations and financial position.

The unaudited pro forma condensed combined financial statements have been prepared based on the aforementioned historical financial statements and the assumptions and adjustments as described in the notes to the unaudited pro forma condensed combined financial statements. The pro forma adjustments reflect transaction accounting adjustments related to the Merger, which are discussed in further detail below. The unaudited pro forma condensed combined financial statements are presented for illustrative purposes only and do not purport to represent the combined company’s consolidated results of operations or consolidated financial position that would actually have occurred had the Merger been consummated on the dates assumed or to project the combined company’s consolidated results of operations or consolidated financial position for any future date or period.

The accounting policies followed in preparing the unaudited pro forma condensed combined financial statements are those used by Omnicom as set forth in Omnicom’s audited historical financial statements. Based on Omnicom’s initial review and understanding of IPG’s summary of significant accounting policies included in IPG’s annual report on Form 10-K for the year ended December 31, 2024, management is not aware of any material differences to conform IPG’s historical financial information to Omnicom’s significant accounting policies. A more comprehensive comparison and assessment will occur, which may result in additional differences identified. Additionally, Omnicom has included certain reclassifications for consistency in the financial statement presentation, including with respect to certain IPG restructuring charges. These reclassifications have no effect on the previously reported total assets, total liabilities and shareholders’ equity, or net income of Omnicom or IPG. See Notes 2 and 3 in the Notes to Unaudited Pro Forma Condensed Combined Financial Statements for more information.

The unaudited pro forma condensed combined financial statements are presented for illustrative purposes only and do not reflect the costs of any post close integration activities, severance, or impairments, or any cost savings or synergies that may be achieved because of the Merger or costs to achieve the synergies. The unaudited pro forma condensed combined financial statements do not give effect to the estimated impacts of the Exchange Offers and Consent Solicitations.

IPG and Omnicom have not had any historical material relationship prior to the Merger. Accordingly, no pro forma adjustments were required to eliminate activities between the companies.

Unaudited Pro Forma Condensed Combined Balance Sheet

As of September 30, 2025

(in millions)

Please refer to the notes to the unaudited pro forma condensed combined financial statements.

Unaudited Pro Forma Condensed Combined Statement of Income

For the Nine Months Ended September 30, 2025

(in millions except share and per share data)

Please refer to the notes to the unaudited pro forma condensed combined financial statements.

Unaudited Pro Forma Condensed Combined Statement of Income

For the Year Ended December 31, 2024

(in millions except share and per share data)

Please refer to the notes to the unaudited pro forma condensed combined financial statements.

NOTES TO UNAUDITED PRO FORMA CONDENSED COMBINED FINANCIAL STATEMENTS

On December 8, 2024, Omnicom Group Inc. (“Omnicom”) entered into an Agreement and Plan of Merger (the “Merger Agreement”), by and among Omnicom, EXT Subsidiary Inc., a direct wholly owned subsidiary of Omnicom (“Merger Sub”), and The Interpublic Group of Companies, Inc. (“IPG”), pursuant to which, subject to the terms and conditions of the Merger Agreement, Merger Sub will merge with and into IPG (the “Merger”), with IPG surviving the Merger as a wholly owned subsidiary of Omnicom. Each share of IPG common stock will be converted into, and represent the right to receive, 0.344 shares of common stock of Omnicom, plus cash in lieu of any fractional shares of Omnicom common stock that otherwise would have been issued. On March 18, 2025, the shareholders of each of Omnicom and IPG approved the Merger. The completion of the Merger is subject to customary closing conditions, including required regulatory approvals. We have secured regulatory approvals for Omnicom’s pending acquisition of IPG in all required jurisdictions and expect to close the Merger by the close of business on November 26, 2025. The Merger is not subject to a financing condition or the completion of the Exchange Offers or Consent Solicitations announced by Omnicom and IPG on August 11, 2025 related to IPG’s outstanding senior notes (the “Exchange Offers and Consent Solicitations”). Omnicom has received sufficient tenders and consents to consummate the Exchange Offers and Consent Solicitations, which are currently set to expire on November 28, 2025.

The unaudited pro forma condensed combined financial statements were prepared in accordance with Article 11 of Regulation S-X. The unaudited pro forma condensed combined financial statements have been prepared from the respective historical consolidated financial statements of Omnicom and IPG and reflect transaction accounting adjustments related to the Merger. The unaudited pro forma condensed combined statements of income for the year ended December 31, 2024 and for the nine months ended September 30, 2025 have been presented as if the Merger had been completed on January 1, 2024. The unaudited pro forma condensed combined balance sheet as of September 30, 2025 has been presented as if the Merger had occurred on September 30, 2025.

Certain balance sheet and statements of income reclassifications have been made to the unaudited pro forma condensed combined financial statements in order to align historical presentation between Omnicom and IPG. Refer to Note 2 and Note 3 for a discussion of these reclassification adjustments.

The unaudited pro forma condensed combined financial statements were prepared using the acquisition method of accounting, and Omnicom is considered the accounting acquirer. The acquisition method of accounting, based on ASC 805, uses the fair value concepts defined in ASC 820, Fair Value Measurement (“ASC 820”). Fair value is defined in ASC 820 as the “price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date.” This is an exit price concept for the valuation of an asset or liability. Market participants are assumed to be buyers or sellers in the most advantageous market for the asset or liability. Fair value measurement for an asset assumes the highest and best use by these market participants, and as a result, assets may be required to be recorded which are not intended to be used or sold and/or at a fair value measurement that does not reflect management’s intended use for those assets. Fair value measurements can be highly subjective, and it is possible the application of reasonable judgment could develop different assumptions resulting in a range of alternative estimates using the same facts and circumstances. ASC 805 requires, among other things, that assets acquired and liabilities assumed in a business combination be recognized at fair value as of the Merger date, with any excess purchase price allocated to goodwill.

As of November 26, 2025, the accompanying unaudited estimated purchase price allocation is preliminary and is subject to further adjustments as additional information becomes available and as additional analyses are performed. The potential changes to the purchase price allocation and related pro forma adjustments could be material.

For the purposes of preparing the unaudited pro forma condensed combined financial statements, Omnicom’s management has conducted a preliminary analysis of the adjustments required to conform IPG’s financial statements to reflect the current accounting policies of Omnicom. This assessment is ongoing, and, at the time of preparing the unaudited pro forma condensed combined financial statements, management is not aware of any material accounting policy differences. Upon consummation of the Merger, Omnicom management will conduct a final review of IPG’s accounting policies to determine if differences in accounting policies or financial statement classification exist that may require adjustments to or reclassification of IPG’s results of operations, assets or liabilities to conform to Omnicom’s accounting policies and classifications. As a result of that review, differences may be identified that, when conformed, could have a material impact on the unaudited pro forma condensed combined financial statements.

The classification of certain balance sheet items presented by IPG under U.S. generally accepted accounting principles (“GAAP”) has been adjusted to align with the presentation used by Omnicom under GAAP, as shown below.

The classification of certain items in the statement of income presented by IPG under GAAP has been adjusted to align with the presentation used by Omnicom under GAAP, as shown below.

Omnicom calculated the total consideration as follows (in millions, except for shares, per share amounts and exchange ratio):

Under the acquisition method of accounting, the total consideration is allocated to IPG’s assets and liabilities based upon their estimated fair values as of the date of completion of the Merger. Based upon the estimated purchase price and a preliminary valuation, the preliminary purchase price allocation is as follows:

The purchase price estimates and the purchase price allocation are preliminary and are subject to change based on the Omnicom share price at the closing date of the Merger, as well as the actual net tangible and intangible assets and liabilities that exist on the closing date of the Merger, and the value of certain tax contingencies. The purchase price allocation related to certain tangible and intangible assets and liabilities is ongoing. This includes, but is not limited to, items such as property and equipment, operating lease right-of-use assets and lease liabilities, and redeemable noncontrolling interests. The final determination of the purchase price allocation will be completed as soon as practicable after the completion of the Merger and will be based on the fair values of the assets acquired and liabilities assumed. The final amounts allocated to assets acquired and liabilities assumed could differ significantly from the amounts presented in the unaudited pro forma condensed combined financial statements.

Refer to Note 5 for additional information on how the consideration and purchase price allocation have been reflected in the pro forma adjustments.

The unaudited pro forma condensed combined financial statements reflects the following adjustments.

The adjustment below reflects the impact to selling, general and administrative expenses (“SG&A”) for those employees whose compensation expense was included within SG&A and Salary and related for those employees included within Salary and related. The following reflects the previously recorded income statement impacts and the adjusted income statement impacts for each period presented.