O'Sullivan claimed an unimaginable F2 victory on the streets of Monaco, starting fifteenth on the grid after a late incident involving Zane Maloney and Joshua Durksen modified the script.
Hadjar was heading in the right direction for victory after controlling the race following Richard Verschoor's mid-race retirement, but became furious and refused to talk to anyone when he saw O'Sullivan exit the pits in front of him.
Knowing that the one likelihood of scoring points on Sunday was a late safety automobile intervention or similar, ART left O'Sullivan on soft tires until lap 40, a risk that was rewarded in essentially the most dramatic fashion as he became the eighth different winner of the season.
Despite Hadjar's fury, the Frenchman moved as much as second within the championship, while third-placed Paul Aron climbed to the highest because of Maloney's inability to attain.
Isack Hadjar, Campos Racing, 1st place on the rostrum
Photo: Mark Sutton / Motorsport images
The start of an F2 feature race is as a rule a quieter affair than a sprint, with drivers confident that pit strategy and safety cars can lift them to the rank.
That was largely evident on Sunday, other than a moment of minor contact between Jake Crawford and Invicta driver Kush Maini within the Grand Hotel turn that eliminated the previous.
The start was also one other nightmare in a difficult season for Alpine-backed Victor Martins. After winning his qualifying group and taking second place on the grid, the ART driver bogged down and dropped like a rock to fifteenth place just before the lights went out.
However, the true drama occurred on lap 19, when Richard Verschoor, leading the race, reported an issue that ultimately ended the race. The problem appeared to limit his Trident's power output entering the race, and although he was able to keep up his position until the pit stops, he was attacked by Hadjar, Paul Aron and the remainder of the sphere shortly after returning to the track.
After leapfrogging Nouvelle Chicane in defense against Aron, Verschoor was handed a five-second penalty, which he served before completing his final tour and retiring.
An additional post-race investigation was announced after he removed the steering wheel and threw it from the automobile before stopping within the pits.
Also within the stoppage phase, Prema, in a duo with Andrea Kimi Antonelli and Oliver Bearman, secured some close motion, going wheel-to-wheel when the previous returned to the track. Ultimately, Bearman sealed the position with a cutting maneuver on the Grand Hotel turn.
Having followed Franco Colapinto for a while, Antonelli was at the middle of the motion again on lap 37 when he made a daring move to steer the MP Motorsport driver past Antony Noghes and climbed to eighth place.
Then got here the twist within the story on lap 40 when O'Sullivan, using an alternate strategy, was given a free stop and Maloney and Durksen got here together because the latter returned to the track from his own stop.
After pitting before the VSC was called, O'Sullivan's stop was deemed legal and allowed the ART driver to steal the victory from Hadjar in essentially the most dramatic of circumstances.
Monaco – Feature race
