*Club domestic league appearances and goals, correct as of 18:44, 28 April 2024 (UTC) ‡ National team caps and goals, correct as of 21:16, 17 November 2024 (UTC)
O'Brien first joined the youth set-up at Accrington Stanley at the age of eight, and went on to captain the under-18s to the fourth round of the FA Youth Cup in 2021–22. He signed his first one-year professional contract in June 2022.[1] On 3 December 2022, he joined Northern Premier League Premier Division club Radcliffe on a one-month loan.[2] He scored on his debut later that day, in a 2–2 draw at Matlock Town.[3] He played a total of seven games for Radcliffe.[3] He made his first-team debut in the English Football League with Accrington after coming on as an 80th-minute substitute for Rosaire Longelo in a 1–1 draw at Port Vale on 7 February 2023.[4] On 13 February 2023, Radcliffe extended his loan by a further 2 months.[5]
On 10 May 2024, O'Brien extended his contract at Accrington Stanley by two years.[6]