O'Brien currently plays for Irish provincial side Connacht in the Pro14, after being promoted from the academy ahead of the 2015–16 season.
[8] After an injury ruled O'Brien out of the 2014 Junior World Cup, he failed to make a senior breakthrough in the 2014–15 season.
[13] In November 2015, O'Brien was named in the Connacht squad for a trip to the Siberian city of Krasnoyarsk for a 2015–16 Challenge Cup game.
[14] He made his senior debut on 14 November in the game against Enisey-STM, coming on after 58 minutes as a replacement for captain John Muldoon.
[15][16] O'Brien made his first league appearance two weeks later in a 2015–16 Pro12 derby against Munster, coming off the bench for Eoghan Masterson as Connacht won at Thomond Park for the first time since 1986.
[23] In April 2014 the Irish squad for the 2014 Junior World Cup was announced, with O'Brien named captain.