Career
He currently plays for the Washington Capitals of the National Hockey League (NHL). O'Brien previously played for the Rimouski Oceanic (2010–12) and Rouyn-Noranda Huskies (2012–14) of the Quebec Major Junior Hockey League. Passed over in the NHL Entry Draft, O'Brien received a training camp invite from the Washington Capitals in 2014, and made the team, earning himself a three-year entry level contract on October 6, 2014.
He made his NHL debut on October 9, 2014 in a game against the Montreal Canadiens, in which he registered his first NHL fight against the Canadiens' Brandon Prust.