Mark Selby became the latest high-profile player to suffer an early exit as Martin Gould clinched a nervy final frame decider.
Gould, who made his highest-ever break in professional snooker of 142 during the match, was pleased to seal victory after surrendering a 3-1 lead.
“He’s one of those players where it doesn’t matter if you’re 3-1 up and 61 in front with 59 on you’d know you’ve got to scrape him off the table,” he said.
“It’s a very pleasing result and it just shows my form is going in the right direction.”
2015 UK Champion Neil Robertson avoided a repeat of his shock first-round defeat in the same event last year by beating Ricky Walden.
Walden did have the chance to cause a shock, drawing level from two frames down but his Australian opponent secured the next two to clinch the match and book a last 16 tie against O’Sullivan.
The pair met just last month in the Quarter-Finals of the Masters with O’Sullivan coming out on top.
China’s Ding Junhui came out on top against his fellow countryman Yu Delu with a 4-2 victory.
Mark Allen put in a blinding performance to thrash Anthony Hamilton 4-0, finishing the match with a 140 break.
Hamilton arrived at the World Grand Prix in good form after beating Ali Carter to win the German Masters last week, which was his ranking title, but wasn’t given an opportunity by Allen.