 Playing major junior in the Ontario Hockey League (OHL), O'Brien was drafted by the Mighty Ducks of Anaheim in the 8th round, 250th overall, in the 2003 NHL Entry Draft.Before earning a full-time roster spot with Anaheim, he played with their minor league affiliates, the Cincinnati Mighty Ducks and Portland Pirates of the American Hockey League (AHL).O'Brien led all AHL defenceman in scoring during the 2006 Calder Cup playoffs with six goals and 22 points in 19 post-season games.