O'Brien began his career with Huddersfield Town, where he was named Academy Player of the Year for the 2017–18 season. After signing a new three-year contract with Huddersfield in May 2018, he moved on loan to Bradford City in August 2018 for the 2018–19 season. His signing was met with some criticism on social media by Bradford City fans. He made his professional debut for Bradford City on 8 September 2018, and later that month was praised by manager David Hopkin. By mid-November 2018 he had become a first-team regular. In December 2018 he was praised by City teammates Hope Akpan, and Jack Payne. Later that month, journalist Simon Parker said that O'Brien was "enjoying an outstanding first season in senior football" and was "a driving force in City's push to get out of the League One drop zone". O'Brien stated playing for City had been "fantastic", that he "felt ready for the challenge", and that he was "happy and determined to enjoy the experience". On 1 January 2019 he scored his first senior goal, in a 3–0 league win at home to Accrington Stanley. In that same match he was later injured and had to be substituted, although the injury was later announced to not be as serious as feared, and he trained in preparation for a league game away at Barnsley on 12 January. In January 2019 it was announced that his loan would continue until the end of the 2018–19 season, after Bradford reached a new deal with parent club Huddersfield. In February 2019, following a poor run of results which saw the club drop to second-bottom in the table, O'Brien spoke about his confidence in the players turning things around. Later that month O'Brien was described by The Guardian as a "smash hit". In March 2019 new manager Gary Bowyer rested O'Brien from first-team duties, ending "a run of 30 successive league starts" in order to be fully fit for the club's relegation battle. Following a return to first-team action, O'Brien spoke about his experience during his first year as a senior professional. In May 2019 he praised David Ball for helping him develop during his time with Bradford, having previously commended Ball for taking him under his wing. In July 2019, following the end of his loan spell with Bradford and upon his return to parent club Huddersfield, O'Brien was given an improved three-year contract. He won the EFL Goal of the Month in January 2020.
Personal life
O'Brien grew up watching Manchester City with his father. During his time with Bradford City, O'Brien used to entertain his teammates by singing.
