O’Brien made his Leinster debut against Cardiff Blues in September 2008 in the Celtic League. He made his Heineken Cup debut against Castres Olympique in December 2008. He did not feature for Leinster in the 2009 Heineken Cup Final, but evidence of O’Brien’s emergence as one of the most exciting young back-row forwards in Europe came in the form of strong interest from France before Christmas of 2010. The Irish Rugby Football Union moved quickly and O’Brien agreed a new three-year deal with Leinster the following January. O'Brien was in outstanding form for Leinster during the pool stages of the 2010–11 Heineken Cup, and was an integral part of the team that beat Northampton Saints to win the 2011 Heineken Cup Final. In 2012 O'Brien helped Leinster win the 2012 Heineken Cup Final, scoring a try as Leinster beat Ulster. In January 2014, O'Brien signed a new two-year deal with Leinster. On 21 December 2015, O'Brien signed a new three-year contract with Leinster.
In 2007, he was a member of the Ireland Under-20 Grand Slam winning squad. O'Brien was called up to the Ireland team for the 2009 November Tests, and he gained his first cap when he came on as a substitute against Fiji at the RDS Arena on 21 November 2009. He also came on as a replacement against South Africa in a 15–10 win on 28 November 2009. He came off the bench against Italy in the 2010 Six Nations Championship, but was dropped afterwards. He was injured during Ireland's 2010 Summer Tests, but won his first start against Samoa in the 2010 Autumn Tests, though he did not feature in any other games. O'Brien started every game for Ireland during the 2011 Six Nations, winning the Man of the Match award against Italy. He came third in the 2011 Six Nations Player of the Championship losing out to the winner Andrea Masi. He was selected in Ireland's final 30-man squad for the 2011 Rugby World Cup in New Zealand. O'Brien started against Australia, Russia, Italy and Wales during Ireland's World Cup campaign, scoring a try against Russia and winning the Man-of-the-Match award against Italy. In the final 2015 Rugby World Cup Pool D match, O'Brien punched Pascal Papé in the first minute of the win over France. The incident went unnoticed by the match officials, and O'Brien later was named man of the match. The win earned Ireland top pool placement, and a quarter-final against Argentina. O'Brien was later cited by World Rugby, and received a one-game ban, meaning he was unable to play in the quarter-final.
In May 2019 it was confirmed that O’Brien would miss the 2019 Rugby World Cup due to a hip injury.
Awards
On 25 May 2011, O'Brien was named ERC European Player of the Year following a season where Leinster were Heineken Cup champions. He was one of five players shortlisted from the 24 Heineken Cup and 20 Amlin Challenge Cup teams that included Leinster teammates Jamie Heaslip and Isa Nacewa. On 15 August 2011, O'Brien was awarded the Guinness/Rugby Writers of Ireland player of the year and received his award at a function in the Aviva Stadium, Dublin.
Personal life
In September 2014 he bought Bellamy’s Pub in Ballsbridge with Leinster team mates Rob Kearney, David Kearney and Jamie Heaslip. After an extensive refurbishment it was reopened as “The Bridge 1859”. Since 2018 O'Brien has been in a relationship with Sarah Rowe.
Controversy
In June 2019, O'Brien was named by the Irish Independent as the Leinster player under investigation following an incident in a Dublin pub on the night of their PRO14 final celebrations; O'Brien was accused of urinating on a member of the public while inside the premises.
