The Cork middleweight was stopped in the opening round of his WBA middleweight world title final eliminator by David Lemieux at the T-Mobile Arena.
The Canadian started purposefully and a left hook landed while O’Sullivan was trying to set up a right hand sent the Irishman down hard.
O’Sullivan attempted to rise but was in an obviously dazed and distressed state, with referee Russell Mora abandoning the count and stopping the fight.
Watch the stoppage and the full fight below:
Video Credit: HBO / RingTV