O'Sullivan, the defending champion, wore strapping on his wrist

There were also defeats for Stephen Hendry, Steve Davis, Peter Ebdon and Paul Hunter but world champion Shaun Murphy battled his into the last 16.

O'Sullivan seemed to be struggling with an injury to his right wrist but refused to confirm he had sustained a fracture while boxing in the gym.

"It's just a little problem the doctors are looking into," he said.

"It's just a bit sore. I did something years ago and it feels a bit achy."

McCulloch, the world number 16 from Preston, had lost his five previous meetings with O'Sullivan but took advantage of the chances that came his way.

I didn't know Ronnie had a problem until I spoke to him at the interval but I don't think it would have made any difference Ian McCulloch

He fired in a 94 clearance to win the third frame and claim a 3-0 lead and secured the sixth for victory with a run of 72 after O'Sullivan ran out of position on 58.

"I didn't know Ronnie had a problem until I spoke to him at the interval but I don't think it would have made any difference," said McCulloch, a semi-finalist in last season's World Championship.

"You have to do what I did, which is clear up when he misses. I've played him several times but haven't often played well against him.

"But it's just one win. I won't get carried away unless I'm here for the final."

Hendry lost 5-3 to Barry Hawkins, Davis was edged out 5-3 by Jamie Cope, Hunter succumbed 5-3 to Anthony Hamilton while Ebdon was thrashed 5-0 by Joe Swail.

Hendry was the victim of a refereeing error in the third frame when he was awarded four points rather than six when Hawkins fouled the pink.

It's weird what that little bit of doubt in your mind can do to you Shaun Murphy

The frame came down to a re-spotted black, which Hawkins potted to lead 2-1 and he pulled away to 4-1 with the help of a 128 break in the fifth.

Hendry's 133 total clearance reduced the gap to two frames, and the seven-times world champion also won the seventh, but Hawkins finished off in style with a well-crafted 86.

Murphy beat Hicks 5-3 but admitted his nerves had started to jangle as the match wore on.

"It's weird what that little bit of doubt in your mind can do to you. You find yourself playing shots you'd never have played before," he said.

Ryan Day, a quarter-finalist at Newport last year, reached the second round with a 5-1 victory over 1991 world champion John Parrott.

Welsh Open scores and results

Day lost the opening frame but compiled two centuries, 122 and 113, as he won five on the trot to set up a meeting with Jimmy White.

Parrott, 41, is down to 44th in the provisional rankings with only the World Championship remaining this season.

The Liverpudlian faces Mansfield's Lee Spick in the final qualifying round on 14 March.