The Carlin driver set a 1m44.597s in his best qualifying so far this season in front of his home crowd, having previously had a best start of eighth.
He sat tenth place in the first half of the session, but on the final runs took pole by 0.03s from Prema Racing’s Arthur Leclerc, while Zane Maloney will line up third for Trident, just 0.014s behind Leclerc.
Team boss Trevor Carlin told Autosport earlier this week that the team had found a breakthrough after struggling in the series for several seasons, which paid off for O’Sullivan who was named Autosport’s BRDC Young Driver of the Year for 2021.
With the track having dried significantly since a damp Formula 1 free practice one session earlier in the day, Ollie Bearman was first to set a flying lap for Prema, with a 1m45.107s putting him 0.2s ahead of Kush Maini in second.
The MP Motorsport driver, who also topped practice earlier on Friday, then improved onto a 1m44.963s before Bearman briefly reclaimed the top spot, but had his time deleted for track limits.
Leclerc put himself into third with a 1m45.221s before the times tumbled as Maloney and Jonny Edgar, having returned this weekend after a period away from racing following a diagnosis with Crohn’s Disease, knocked him into fifth.
The field pitted around the halfway mark, with just one second separating the top 15 as Maini led from Bearman and Maloney.
As they returned to the circuit, the lead quickly changed hands, with Caio Collet briefly heading the standings for MP Motorsport before O’Sullivan took the top spot.
Collet will start fourth for Sunday’s race, with Prema’s Jak Crawford and Bearman rounding off the top six, marking a change in the qualifying fortunes of the Italian outfit so far this year.
Roman Stanek, who sits second in the championship, will start eighth, while frontrunner Victor Martins starts 11th for ART Grand Prix.
Despite his early pace, Maini will line up tenth, putting him on reverse-grid pole for Saturday’s sprint race, while Isack Hadjar starts second for Hitech.
Edgar also struggled to improve in the second half of the session, and will start 14th.
