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.