On October 6, 2008, O'Brien was traded with forward Michel Ouellet to the Vancouver Canucks for defenceman Lukáš Krajíček and prospect Juraj Šimek.

Pavelski allegedly speared Canucks forward Daniel Sedin at the end of the game, a 2–1 overtime loss, leading O'Brien and teammate Willie Mitchell to intervene.

[9] Soon thereafter, O'Brien garnered significant media attention when, after being unexpectedly made a healthy scratch, he claimed that general manager Mike Gillis implied in a one-on-one meeting that he is only interested in him fighting.

[12] Becoming a restricted free agent in the off-season, O'Brien re-signed with the Canucks to a one-year, $1.6 million contract after turning down the arbitration process.

[20] Acquired 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.

[21] 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.

[24] 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.

[27] As a free agent O'Brien failed to attract significant interest and, on September 2, 2014, he accepted a professional try-out contract to the Florida Panthers training camp.

[29] On July 16, 2015, O'Brien signed as a free agent to a one-year, two-way contract in a return his original NHL club, the Anaheim Ducks.

[30] Unable to make the Ducks roster out of training camp, O'Brien added a veteran presence to AHL affiliate, the San Diego Gulls, for the duration of the 2015–16 season, posting 1 goal and 17 points in 57 games.

[31] Midway into the following 2017–18 season, O'Brien was signed to strengthen the blueline of Austrian club, EHC Black Wings Linz of the EBEL, on November 9, 2017.