 On July 13, 2011, he was signed to a one-year contract with the Colorado Avalanche.Acquired to initially to provide depth insurance to the defense, O'Brien impressed out of training camp and made his Avalanche debut on the opening night of the 2011–12 season in a 3-0 defeat to the Detroit Red Wings.With his experience, physicality and leadership he quickly became a fixture on the defense of a youthful Avalanche team and appeared in his 400th career game in a 4-1 loss to the San Jose Sharks on November 20.