O'Brien played junior hockey for the United States National Under-18 Team in the NAHL from 2004 until 2006. He then entered the University of Minnesota and played a season of hockey for the Golden Gophers as a 17-year-old. He had to receive an exception to play in university hockey; he was the youngest player in UScollege ice hockey. O'Brien was drafted 29th overall by the Ottawa Senators in the 2007 NHL Entry Draft. O'Brien played defense before switching in high-school to "play as a power-forward center-ice man" His final Central Scouting rank was 38th. He was picked by Ottawa general managerBryan Murray because "size matters". O'Brien is a strong skater, he blocks shots and he is a penalty killer. It was the third year in a row that the Senators picked a US-born player. They picked Nick Foligno 28th overall during the 2006 NHL Draft, and Brian Lee ninth overall in 2005. O'Brien played the following 2007–08 and 2008–09 seasons with the major-junior Seattle Thunderbirds of the Western Hockey League. On March 30, 2009, he turned professional, joining the Binghamton Senators, Ottawa's AHL affiliate. 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. He played a total of six NHL games with Ottawa during the 2010–11 NHL season, registering 11 shots on goal, no points, and two penalty minutes. O'Brien was returned to Binghamton and was a member of the 2011 Calder Cup championship team. O'Brien returned to Binghamton for the 2011–12 season after attending Ottawa's training camp. He was called up to Ottawa in February 2012 and scored his first NHL goal on February 15, 2012, against the Florida Panthers. On July 18, 2012, the Ottawa Senators announced that they resigned O'Brien to a two-year contract. At the conclusion of his second contract with the Senators, O'Brien was not offered a new contract and signed his first contract abroad with in the Russian Kontinental Hockey League with Metallurg Novokuznetsk. 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. On December 26, 2014, O'Brien signalled a return to the AHL in signing with the Wilkes-Barre/Scranton Penguins. He was then immediately claimed off waivers by the Hershey Bears. On July 1, 2015, O'Brien earned a NHL contract in signing as a free agent to a one-year, two-way deal with the New Jersey Devils. In the 2015–16 season, O'Brien compiled his most productive season in the AHL since 2011, in recording 19 goals and 38 points with the Albany Devils. He was recalled by New Jersey to appear in 4 scoreless games with the Devils. At the conclusion of the season, O'Brien left the Devils organization as a free agent. On July 1, 2016, he signed a one-year, two-way contract to join the Colorado Avalanche. O'Brien was assigned to bolster AHL affiliate, the San Antonio Rampage for the 2016–17 season. 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. He was briefly recalled by the Avalanche, but was returned to the AHL without appearing for the club. 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. 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. On February 25, 2018, the Ottawa Senators signed O'Brien to a two-year, two-way contract and was then assigned to the Belleville Senators for the remainder of the 2017–18 season. O'Brien was called up to Ottawa on February 26, 2018 after Nick Shore was traded. O'Brien appeared in 10 games, registering 1 assist in his return to the NHL. In the following season within the Senators organization, O'Brien's 2018–19 campaign was limited to just 11 games with the Belleville Senators due to injury. 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.
Personal life
O'Brien was among a small group of Senators' players who had planned on attending the 2013 Boston Marathon. 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. "Sure enough I wake up from a nap and have a bunch of text messages. I turn on the TV... it was just a twist of fate. It's definitely something that's hard to think about it. Thank goodness we didn't go", said O'Brien.
