O'Sullivan defeated Matthew Hall in a close contest, at Upton Park, London, on 14 July 2012 on the undercard of David Haye vs. Derek Chisora, to win the WBO intercontinental middleweight title.
[4] After a loss to Chris Eubank Jr., O'Sullivan won four straight fights to earn a December 2017 bout against rising prospect Antoine Douglas on HBO.
[5] After beating Berlin Abreu on 4 May, it was announced that O'Sullivan would be fighting on the undercard of Canelo Álvarez vs. Gennady Golovkin II on 15 September.
[8] On 11 January 2020, O'Sullivan fought former world champion Jaime Munguia, who was ranked #1 by the WBO and #3 by the WBC at middleweight.
O'Sullivan put up a great fight, but in the end Munguia just had too much left in the tank, as he finished the Irishman in the eleventh round via TKO.