Born in Chelsea, London, O'Sullivan moved to Cheltenham at age eight. He took up motorsport at nine and began competitive karting in 2014, winning the British Super One Rookie Championship in his first year. He progressed through the national Super 1 series and in 2017 made his first European appearance, finishing twelfth in the CIK-FIA European Championship with Ricky Flynn Motorsport. In 2018 he achieved his best international karting result, finishing second in the German Karting Championship's junior class, and made a sole appearance at the Karting World Championship.
O'Sullivan made his car racing debut in the 2019 Ginetta Junior Championship with Douglas Motorsport. A mid-season switch to R Racing produced three victories at Snetterton and Thruxton and a nine-race podium streak, lifting him to second in the standings as the highest-placed rookie.
For 2020 he joined Carlin in the F4 British Championship alongside Christian Mansell and Matías Zagazeta. He won the opening race at Donington Park and added two victories at Brands Hatch, but lost the championship lead to Luke Browning after Browning swept the Oulton Park and Knockhill rounds. O'Sullivan recovered with nine consecutive podiums including four race wins. At the final round he took the chequered flag but a red-flag ruling meant half-points were distributed, dropping him three points behind Browning. He was classified vice-champion.
Remaining with Carlin for the 2021 GB3 Championship, O'Sullivan took an early lead after winning at Brands Hatch in round one. He withstood threats from Reece Ushijima and pulled away by sweeping the first two races at Donington Park from pole. Further victories at Oulton Park and Silverstone allowed him to clinch the title at the final round in dominant fashion. For this campaign he received the Aston Martin Autosport BRDC Award in February 2022, announced ahead of Jonny Edgar, Louis Foster and Oliver Bearman, and was presented the BRDC's Jim Clark Trophy for the GB3 title.
O'Sullivan joined Carlin for the 2022 FIA Formula 3 Championship. Despite an inconsistent season hampered by incidents and penalties, he scored two podiums and ended eleventh in the standings with 54 points. He won the Comeback of the Year award at the F3 Prize Giving ceremony for his charge from the back to fourth in Budapest. For 2023 he moved to Prema Racing. He won the Melbourne sprint race after Franco Colapinto's disqualification promoted him to first, then added a Barcelona sprint win fending off fellow Williams Academy member Luke Browning. In Austria he took his first Formula 3 feature race win. A dominant victory from pole in Hungary equalled the F3 win record with his fourth win of the season, pushing him to second in the championship. He finished as runner-up with 119 points, four victories, one additional podium and one pole position.
In February 2022 O'Sullivan was confirmed as a member of the Williams Driver Academy. As part of his Autosport BRDC Award prize he drove the Aston Martin AMR21 at Silverstone in late 2022. He made his Formula One practice debut driving the Williams FW45 in FP1 at the 2023 Abu Dhabi Grand Prix, finishing eighteenth, and later completed 50 laps in the post-season young drivers' test. He departed the Williams Academy after 2024.
O'Sullivan stepped up to Formula 2 with ART Grand Prix for 2024, partnering 2022 F3 champion Victor Martins. ART struggled with the new-specification car throughout the season. His most notable result came in Monaco, where starting fifteenth he ran long on hard tyres and pitted just before a virtual safety car on lap 40, emerging ahead of the entire field including net leader Isack Hadjar to take his first Formula 2 win. He also won a rain-shortened sprint in Spa-Francorchamps from reverse pole. O'Sullivan withdrew from the remaining three rounds after Monza due to funding difficulties, ultimately placing sixteenth in the standings with two wins and 59 points.
After testing at Suzuka in December 2024, O'Sullivan joined Kondo Racing for the 2025 Super Formula Championship. He switched to the single-car Team Impul entry for 2026. Concurrently he made his Super GT debut in the GT300 class with new entrant CarGuy MKS Racing in 2025, driving a Ferrari 296 GT3 alongside Rikuto Kobayashi, and continued with the team in 2026. He also participated in Formula E rookie practice sessions with Envision Racing and was named their simulator driver for the 2024–25 season.
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