2009-10: O'Brien played junior hockey for the Dixie Beehives in the Central Canadian Hockey League. In 44 games he scored 3 goals with 4 assists and had 45 penalty minutes. The Etobicoke-based team won just 12 games and finished 11th in the 12-team West Division.
2010-11: After passing through the OHL draft, O'Brien signed with Chicoutimi in the Quebec Major Junior Hockey League. He skated in 55 games for the Sagueneens, seeing some time at both forward and defense. He scored 1 goal with 9 assists and was minus-nine with 33 penalty minutes. After finishing fourth in the East, Chicoutimi was swept by Drummondville in the first round of the playoffs. In four playoff games O'Brien scored 1 goal with 1 assist and was an even plus/minus with 6 penalty minutes.
2011-12: O' Brien was a physical force on the blue line for Chicoutimi while providing timely offense in his second QMJHL season. He was at his best in the playoffs when the Sagueneens made a run to the QMJHL semifinals. In 68 regular season games he scored 8 goals with 21 assists and was plus-two. O'Brien was third on the Sagueneens with 95 penalty minutes. After finishing fifth in the East Division, Chicoutimi knocked off eventual Memorial Cup champion Shawinigan in a second round series and handed Saint John it's only loss in 17 playoff games. O'Brien scored 1 goal with 9 assists and was minus-one with 31 penalty minutes in 18 playoff games. Though not among the 210 North American skaters listed in Central Scouting's final rankings, he was selected by Anaheim in the fourth round (108th overall) of the 2012 NHL Draft.
A rugged blue-liner who was passed over in his first year of draft eligibility, O'Brien doesn't back down from a challenge and has the size to back that up. A hard-nosed and punishing player, he also has some offensive ability. But he needs to continue to polish his play in his own end, pick his spots better and most importantly, improve his skating.