O'Sullivan vs. Hall

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.^[2]

O'Sullivan vs. Nunez

It was announced following this defeat of Larry Smith that O'Sullivan would take on former world contender Milton Nunez.^[3] O'Sullivan, before the fight, described the bout as "the most important fight of my life". He started the fight well with some solid body shots easily claiming the first round. He knocked Nunez down in the second round and ultimately scored an impressive victory, by TKO, over Nunez in the third round.^[4]

O'Sullivan vs. Douglas

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. While the fight was meant to be a showcase for Douglas, O'Sullivan won a gritty battle by seventh-round knockout.^[5]

O'Sullivan vs. Lemieux

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. A fight with David Lemieux, a fight that Spike had previously chased in recent years, was mooted but O'Sullivan seemed reluctant to give Lemieux the fight as he had previously turned it down himself. However, after weeks of back and forth verbal jousting online, the fight was officially made with both fighters declaring that they will knock the other out and many tipping it as a potential fight of the night.^{[citation needed]}

O’Sullivan was knocked out in the first round.^[6]

In his first comeback fight, O'Sullivan defeated Gabor Gorbics on points in his home country of Ireland.^[7]

In his next fight, O'Sullivan recorded his second win in a row, defeating overmatched opponent Khiary Gray in six rounds.^[8]