O'Riley joined the academy of Fulham at the age of eight, having previously played for Teddington-based NPL Youth.[3] He progressed through several age groups, eventually becoming a regular in the club's U18 and reserve sides.[4] He made his first team debut on 8 August 2017 in a 2–0 EFL Cup first round away win over Wycombe Wanderers.[5]
Over the next two seasons O'Riley featured for the club's U21 side in several EFL Trophy group matches whilst on the fringes of the first team. He made his Championship debut on 1 January 2020 as a 19th-minute substitute in a 2–1 home defeat to Reading.[6] O'Riley left the club in the summer of 2020, having declined a new three-year first team deal amidst rumoured interest from several clubs domestically and abroad.[7]
Milton Keynes Dons
On 24 January 2021, O'Riley signed for League One club Milton Keynes Dons, having spent six months training with the side as a free agent since leaving Fulham.[4] He made his debut for the club two days later on 26 January 2021, in a 1–0 home defeat to Charlton Athletic.[8] Just a week after joining the club, O'Riley scored his first professional career goal in a 2–0 away win over AFC Wimbledon on 30 January 2021.[9]
After an impressive start to the 2021–22 season, on 11 December 2021 he was named EFL Young Player of the Month for November 2021, a month in which he scored four goals in as many appearances.[10]
Celtic
On 20 January 2022, O'Riley joined Scottish Premiership club Celtic on a four-and-a-half-year deal for a reported £1,500,000 fee after the club met his release clause.[11][12] He made his debut on 26 January 2022, starting in a 2–1 away win over Hearts.[13] On 9 February 2022, O'Riley scored his first goal for the club in a 3–2 away victory over Aberdeen.[14] On 9 April 2022 he scored a brace in a 7–0 home win over St Johnstone.[15] O'Riley would help Celtic lift the 2021–22 Scottish Premiership that year.[16]
On 6 September 2022, O'Riley made his UEFA Champions League debut in a 3–0 home defeat against Real Madrid.[17] In September 2023, he signed a new four-year contract with the club.[18] On 20 October 2023, O' Riley was named Scottish Premiership Player of the Month for the month of September. In April 2024, he was named in the PFA Scottish Premiership Team of the Year for the 2023–24 campaign.[19] A month later he was voted Player of the Year, Young Player of the Year and Players' Player of the Year for the 2023–24 season at the club's annual awards ceremony, topping off a successful season in which he went on to claim his third successive league title with Celtic.[20]
Brighton & Hove Albion
On 26 August 2024, O'Riley joined Premier League club Brighton & Hove Albion for a fee of £25 million, signing a contract until June 2029.[21] He made his debut the next day, starting in a EFL Cup tie against Crawley Town, but after a nasty tackle was injured nine minutes into the game and forced to come off.[22] O'Riley subsequently underwent ankle surgery, with Brighton chairman Tony Bloom confirming that it was hoped he would return to full fitness "at some point in the latter part of the year."[23]
International career
O'Riley was an England youth international. As well as England, he is eligible to represent Denmark due to his mother's side of the family.[24] In February 2022, O'Riley declared his interest in representing Denmark internationally when he stated that: "I played for England youth teams but at the same time, I do feel quite Danish. My mum is Danish and I can speak a decent amount and I can understand a very good amount. In that sense, I don't think it is out of the question at all. If I did get a call up from Denmark, I don't think I'd be saying no".[25]