[2] Having taken up the sport when he was just nine years old,[3] O'Sullivan started his competitive karting career in 2014, where he won the British Super One Rookie Championship.

[10] He started the season off strongly, with a win at the season-opening round at Donington Park and two victories at the Brands Hatch Grand Prix Circuit.

[20] A pair of victories from Reece Ushijima at the next round put O'Sullivan's lead under threat,[21] but the Brit was able to get away from his championship rivals by winning the first two races of the third weekend at Donington Park, having started both from pole position.

[29] For his efforts, O'Sullivan was nominated for the 2021 Autosport BRDC Award, and in February 2022, received that honour ahead of Jonny Edgar, Louis Foster and Oliver Bearman.

[36] O'Sullivan had a poor round in Barcelona scoring no points,[37] but bounced back in Silverstone with an unexpected pole position, describing it as "really nice".

[38][39] He would lose his lead to Arthur Leclerc on Sunday, but was able to narrowly hold off Ollie Bearman for second, scoring his first podium in the category.

[42][43] He had a disappointing Spa-Francorchamps round despite starting from reverse pole, as he damaged his front wing during a first lap battle with Juan Manuel Correa.

[44] After qualifying tenth and starting third in the sprint at Zandvoort O'Sullivan took his second podium which ended up being his final points finish of the season.

[58] In Barcelona, O'Sullivan qualified on reverse pole, and would fend off fellow Williams Academy member Luke Browning to secure his second win.

[62] Sunday was much better, O'Sullivan mastered his way into the lead, taking advantage of his rivals fighting and breakthrough for his first Formula 3 feature race win.

[66][67] His sprint race was compromised by making contact with Nikola Tsolov, which forced the Briton to pit for a new front wing and was classified 21st.

[68] He then went on to dominate the feature race from pole, equalling the F3 win record with his fourth victory, propelling him to second in the standings.

During the sprint race, he impressed by finishing fourth, but was penalised for passing Hugh Barter by gaining time off-track, dropping to 15th.

Due to this, he was able to jump the entire field, including net race leader Isack Hadjar to secure his first Formula 2 win.

[86] In Silverstone, O'Sullivan was making a charge in the sprint from 14th and made his way to the points until he tipped teammate Martins into a spin, taking both out of the race.

[93] Media coverage suggested the end of O'Sullivan's championship was connected to an issue with ART[94] but he has since confirmed it was purely down to funding, and he had a great relationship with the team.

[1] During late October, O'Sullivan got his first taste of F1 machinery, driving the Aston Martin AMR21 at the Silverstone Circuit as part of his reward for claiming the Autosport BRDC Award the previous year.

[98][99] O'Sullivan debuted in Formula One machinery while driving the Williams FW45 during free practice (FP1) at the 2023 Abu Dhabi Grand Prix.

[102] O'Sullivan is set to make his Formula E debut during the rookie free practice session at the Diriyah ePrix, driving for Envision Racing.