The Williams junior put in a masterclass to seal a fourth win of the season and jump up to second in the championship standings.
He crossed the line 2.3s ahead of rookie team-mate Dino Beganovic, with fellow Williams Academy member Franco Colapinto in third for MP Motorsport.
Having lined up on pole, 2021 Aston Martin Autosport BRDC Young Driver of the Year Award winner O’Sullivan held off his rivals and immediately set about building a gap to Ferrari junior Beganovic.
It was as high as 4.2s by the mid stages, and though Beganovic was able to cut it by two seconds, it was the Briton who took the win.
The race was shortened by five laps – from 24 to 19 - on Saturday evening due to “safety reasons” after a report from Pirelli recommended that the distance was shortened due to the level of tyre wear experienced in Saturday’s sprint race.
The top 10 remained unchanged off the line in a relatively sedate start for F3, while further back Pepe Marti (Campos) took two places by lap two to move into 11th.
The Spaniard took 10th on lap four from Nikola Tsolov (ART) while ahead of them, championship leader Gabriel Bortoleto passed Jonny Edgar for seventh in an identical move at Turn 1 with the help of DRS.
Marti continued his steady climb over the next two laps, taking as many positions when he passed Campos team-mate Christian Mansell and Edgar.
Colapinto took third from Leonardo Fornaroli (Trident) on lap nine at Turn 1, with his team-mate Oliver Goethe holding onto the pair in the podium fight.
Goethe then took fourth at Turn 2 the following lap before Paul Aron (Prema) also passed Fornaroli two tours later, the Italian dropping back as Aron sealed fifth.
Source: Autosport