Victory at the Dafabet English Open last month set up the possibility of the 41-year-old completing a clean sweep of the four 'Home Nations' events, for which the seven-figure bonus cheque was on offer.
But a 4-1 loss to Gateshead-born Slessor in the third round in Belfast ended the prospect of O'Sullivan landing the richest prize the sport has ever offered.
Slessor was on offer at 8/1 with Sky Bet to beat O'Sullivan and a 4-1 win to the outsider was priced up at 40/1.
To land the big bonus, O'Sullivan would have needed to win this week, add the Scottish Open title in December and also triumph at the Welsh Open in March. But such has been the strength of O'Sullivan's game in recent weeks that it was beginning to look an achievable goal.
Slessor's biggest career scalp came hours after he checked out of his hotel, half-expecting to lose.
The 23-year-old said on Eurosport: "To play Ronnie was just a dream anyway, you know the crowd's going to be full. I just went in not expecting anything. All I wanted to do was try my best and have a good attitude and see how far it got us.
"We checked out this morning just in case because we've got a flight tonight at half eight, which obviously I can't make now so I need to try to find a hotel."
Assessing his win, Slessor had his say on recent comments from O'Sullivan regarding the strength of the game down the rankings.
O'Sullivan has claimed the tour's lower reaches are populated by "numpties", but Slessor said: "I know everybody can play on their day. As Ronnie says... there's not as many numpties as he probably thinks there is."
There was a second major shock in the third round, with Gary Wilson ousting four-time world champion John Higgins in an entertaining contest.
Wilson, from Wallsend, traded blows with his decorated opponent throughout. Higgins pieced together a hat-trick of century breaks - 142, 120, 135 - for a 3-1 lead but 32-year-old Wilson rallied for a stunning victory.
However he fell in the next round to Li Yuan 4-1.
Slessor was also a winner in the fourth round as he beat Sam Craigie 4-1, and faces Li next. They are joined in the last eight by Robert Milkins, Tian Pengfei, Lyu Haotian, Mike Dunn, Mark Williams and Yan Bingtao.
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