The Rocket was one of several upsets at Belfast's Waterfront Hall, with only five of the top 16 players advancing.

The carnage began on Sunday when Mark Selby, Neil Robertson, Xiao Guodong, Shaun Murphy and Ali Carter were all eliminated, before Chris Wakelin and Barry Hawkins joined them on Monday.

Judd Trump was edged out in a deciding frame by world No56 Martin O'Donnell.

O'Sullivan's defeat was particularly painful given how convincingly he had seized control of the match.

After Cheung took the opening frame, the 50-year-old rattled off three consecutive frames with ruthless efficiency, compiling breaks of 56, 138 and 72.

But the Rocket's momentum stalled in the fifth frame when he squandered a promising opportunity on a red, allowing Cheung to capitalise with an impressive 88-point run.

The 27-year-old's confidence surged as he levelled the contest with a break of 77 to force a decider.

O'Sullivan made another costly error in the final frame, handing Cheung the advantage, and the qualifier sealed a maiden main-draw ranking event victory with a composed break of 61.

Speaking afterwards, Cheung told the BBC: "I'm very happy that I can still play well under pressure.

"To be honest, as you can see in the arena, Ronnie had a lot of fans. I was actually scared a little bit before the match, but this is the best experience.

"No matter if I had lost or won, I think it is the best experience for my snooker career and that is all I need.

"To play the legend of the game and to win, this is very satisfying for me."

Cheung now faces Stuart Bingham for a place in the last 16.

Trump's elimination followed a similarly dramatic pattern.

O'Donnell took the lead on three separate occasions, only for the world No3 to respond each time with breaks of 91, 94 and 52.

However, O'Donnell, who registered three half-century breaks of his own, held firm in the decider to record his first-ever victory over the former world champion.

Defending champion and former world champion Kyren Wilson made it through, dispatching Ben Mertens 4-0 in a comfortable progression to round two.

Luca Brecel also advanced with a 4-1 victory over Lei Peifan, while John Higgins beat Liam Highfield.

For O'Sullivan, ranked 13th in the world, the loss marked a disappointing return to domestic competition.

He had not featured at a UK tournament since his narrow 13-12 second-round defeat to Higgins at April's World Championship, and was appearing at the Northern Irish Open for the first time since 2022.