 His first goal for the Drogs was the winner in a 2–1 win over Galway United on 26 May 2011, their first win of the season.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.He joined Dundalk on the final day of the 2012 July transfer window.