O'Brien made his National Hockey League debut on December 31, 2010, dressing for the Senators in an away game against the Columbus Blue Jackets.

[11] In the 2014–15 season, O'Brien scored a respectable 12 points in 22 games before he opted for a return to North America and secured a release from Metallurg Novokuznetsk on December 14, 2014.

[12] On July 1, 2015, O'Brien earned an NHL contract in signing as a free agent to a one-year, two-way deal with the New Jersey Devils.

[15] Hampered by injury throughout the campaign, O'Brien was unable to continue his scoring pace with the Rampage, collecting 9 goals and 24 points in 53 games.

As an un-signed free agent over the summer, O'Brien opted to return to familiar settings in accepting a professional try-out contract to start the 2017–18 season with inaugural AHL club the Belleville Senators, affiliate to the Ottawa Senators.

[16] After playing in the opening four games of the season, O'Brien's try-out was successful as he secured a one-year AHL deal with Belleville on October 19, 2017.

As an impending free agent from the Senators, O'Brien opted to return abroad in securing a one-year contract with German club, the Thomas Sabo Ice Tigers of the DEL, on June 18, 2019.

O'Brien and the other players had originally intended to be at the marathon's finish line during the time at which bombs exploded, killing and injuring several spectators and runners.

Scratches for that night's game against the Boston Bruins, the players changed their plans at the last minute and elected to return to their hotel for a nap instead.