 Affected by the lockout-shortened 2012–13 season, O'Brien was unable to emulate his previous year as he was relegated primarily as a depth defenseman to register only 4 assists in 28 games.On June 27, 2013, O'Brien was traded by the Avalanche along with David Jones to the Calgary Flames in exchange for Alex Tanguay and Cory Sarich.On January 25, 2014, O'Brien was placed on waivers by the Flames.