The former accountant will have an equity stake in the fledgling Passion & Partners along with the creative chief Simons expects to announce next week.
O'Hanlon's appointment is the result of a headhunt brief put out by Simons, Ogilvy's former group chief executive, for a planner with a business background.
"People like David who know about business rather than just being locked into advertising are quite rare," Simons said. "Not only is he a grown-up but, having worked in a place like HHCL, he has the advantage of being involved at the cutting edge of communication."
O'Hanlon will be one of four partners in the venture, which will open its doors for business within the next fortnight. The others include Simons and Bruce Raynor, a management consultant who worked with Simons on the development of More Th>n for Royal & Sun Alliance.
Simons said that the venture would "combine the muscle of strategic consulting with a broad church of creative excellence".
O'Hanlon is a philosophy graduate who left accountancy to join the then Still Price Lintas as a planner in 1992. He switched to HHCL the following year after a brief spell at J. Walter Thompson.
He worked on HHCL's acclaimed Tango campaign for four years and on the origination of the Ronseal "does exactly what it says on the tin" advertising.