The Cork middleweight took on Canadian former IBF champ David Lemieux, his Golden Boy stablemate, at the T-Mobile Arena in a WBA World Title Final Eliminator and was stopped in the very first round.
A huge left hook from Lemieux put the Celtic Warriors fighter down and out in what was one of the chief support bouts on the Gennady Golovkin v Canelo Alvarez rematch undercard.
34-year-old O’Sullivan came into the fight on the best run of form of his career folloing a series of wins which saw him establish a place in the Golden Boy stable. Having stopped Nick Quigley, the Leesider then upset Antoine Douglas and destroyed Berlin Abreu in successive fights.
This run, however, would end violently at the gloves of Lemieux.
Ahead of the bout there had been question marks surrounding the 29-year-old. The Québécois fighter was utterly dominated by Billy Joe Saunders in a world title challenger last December before missing weight badly for his comeback bout versus Karim Achour – and having looked quite drawn on the scales yesterday, subsequently rehydrating almost 20lbs.
The Montreal man, though, was just too big and too powerful for O’Sullivan and ended the bout after 2 minutes and 44 seconds of the first round.
Lemieux, who had shared a heated build-up with the Mahon man, began extremely aggressively in the opener as O’Sullivan looked to catch and counter.
A jab from O’Sullivan knocked the Canadia momentarily off balance but this would be the only bright spot for the Irishman who was dropped heavily by a left hook while throwing a jab.
Down on his knees, his first ever career trip to the canvas, a dazed O’Sullivan was halted by referee Russell Mora whose count only reached ‘FIVE’ with Spike’s eyes gone and obviously in no position to continue.
The loss sees O’Sullivan’s record fall to 28(20)-3(2) while Lemieux improves his record to 40(34)-4(2).
The big puncher – who was also dominated by Golovkin in a unification back in 2015 – will now look for another world title shot.
O’Sullivan, who received the highest purse of any fighter on the undercard at $400,000, will enjoy some time off with family before making a decision on his next move.