He will take over the day-to-day running of the business, which is the biggest Whybin\TBWA office in Australia.
Before moving to the agency, he was the managing director of M&C Saatchi in New Zealand.
O'Neill said: "This is an irresistible opportunity. The TBWA network is the best in the world. Plus, I get to team up with Scott Whybin and a world-class creative department."