(born August 9, 1983) is a Canadian former professional ice hockey defenceman, coming from a lineage of hockey players. His father, Pat O'Brien, played junior in Canada, while his uncle, Dennis O'Brien, had a 10-year career in the NHL, mostly with the Minnesota North Stars.
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. The Pirates had made it to the semi-finals where they were defeated in seven games by the Hershey Bears. In the off-season, he was re-signed by the Ducks to a one-year contract on August 3, 2006.
O'Brien made his NHL debut against the Los Angeles Kings on October 6, 2006. He scored his first NHL goal later that month against Dominik Hašek in a 4–1 win over the Detroit Red Wings on October 18. He completed the night with a Gordie Howe hat trick by getting an assist and a fight against Brad Norton. On November 8, he scored a goal and two assists against the Vancouver Canucks for a three-point night.
At the trade deadline on February 24, 2007, O'Brien was dealt with a third-round draft pick in 2007 to the Tampa Bay Lightning for goaltender Gerald Coleman and a first-round draft pick in 2007. He had established himself as a fighter during his rookie season. At the time of his trade, he was ranked fifth in the league in fights with 12. Making the transition to Tampa Bay, however, his role as an enforcer was diminished as the club was less prone to fight than the Ducks had been. He made his NHL playoff debut with the Lightning on April 12, meeting the New Jersey Devils in the first round.
After finishing his rookie season with a combined total of two goals and 16 points in the regular season between Anaheim and Tampa Bay, he was re-signed by the Lightning in the off-season.