According to the Wall Street Journal, an amendment complaint was filed last month in federal court in Manhattan by Tierney, the former chief executive of Seneca. He claims that Omnicom owes him $1m (£511,000) in stock options, a claim denied by Omnicom, which insists that the company was a separate entity from the network.
Tierney claims that Omnicom created Seneca in 2001 to move ailing internet assets off its balance sheet.
In a statement, Omnicom Group lawyer Peter Wald said: "We strongly dispute Mr Tierney's contention that Omnicom controlled Seneca.
"The evidence shows that Mr Tierney and his management team have been responsible for the successful day-to-day activities of Seneca since May 2001."
Tierney's complaint states that Seneca was under control of the network and its president and chief executive, John Wren, and that "Omnicom often made major decisions regarding Seneca and its assets" without informing Tierney. He bowed out as chief executive of the company in 2004.
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