He began his senior career at Shamrock Rovers making his debut on 7 May 2003 in a League of Ireland Cup tie against Bray Wanderers.
O'Brien then signed for bitter rivals Bohemians[3] for the 2005 season and he made his debut for Bohs against his former club Shamrock Rovers on 18 March 2005 at Dalymount Park.
[4] He never showed the same form at Bohemians and midway through the 2006 season, he returned to Shamrock Rovers on loan where he helped the Hoops to the First Division title.
In need to restart his flagging career, O'Brien departed Bohemians to sign on loan for nearby rivals Shelbourne in July 2007.
After nailing down a starting place in Mick Cooke's midfield, O'Brien signed a new one-year contract for the Drogs, keeping him at United Park until the end of the 2012 season.