O'Brien returned from injury ahead of the 2015–16 season. He featured for Connacht in their preseason match against Grenoble, and was made captain of the Eagles side. 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. 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. 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. In January 2016 it was announced that O'Brien had signed a senior contract with contract, which sees him become a member of the full squad for the 2016–17 season.