O'Sullivan completed a comfortable 6-2 win over Marcus Campbell without ever finding his best form with former UK holder Ding Junhui making light work of Fergal O'Brien in a 6-1 success.
"Marcus didn't play anywhere near his best today and neither did I," said O'Sullivan.
"We both had to be patient and wait for something to happen. "I potted some good balls and made some breaks so there was a bit of fluency there but you could see Marcus was struggling; he can play a lot better than that."
Ding is bidding for a fourth straight ranking event success, and will meet Ricky Walden in the last 16.
Stephen Hendry was the last man to win four straight ranking titles back in the 1990s.
World number Neil Robertson was another comfortable victor in seeing of China's Li Hang 6-3 to set up a meeting with his practice partner Joe Perry.
The Sheffield amateur Mitchell Travis is out of the tournament after a 6-1 defeat to David Morris.
Defending champion Mark Selby will face Graeme Dott for a place in the quarter-finals after he completed a 6-2 win over Stuart Carrington of Grimsby.
O'Sullivan meets Robert Milkins in the last-16 on Wednesday.