Omnicom has closed its acquisition of rival Interpublic in an all-stock deal reshaping the advertising and marketing sectors and creating the world’s largest ad holding company.

The combined company, with a pro forma combined revenue in excess of $25 billion, will trade under the OMC ticker symbol on the New York Stock Exchange. John Wren remains Chairman & CEO. The full management team will be unveiled on December 1.

The transaction, valued at around $13 billion, was announced last December. Interpublic shares at that point jumped 11% on the news, while Omnicom stock slid 7%, with the moves emulating the classic pattern of a merger.

Of the four major holding companies, Omnicom and Interpublic are Nos. 3 and 4, respectively, based on revenue. Only Publicis and WPP are larger. The m

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