The four-time winner beat Marcus Campbell 10-4 at the Crucible in his first competitive match since ending a 12-month sabbatical and will now take on fellow Essex player Ali Carter, who he beat 18-11 in last year's final, or tournament debutant Ben Woollaston.
O'Sullivan is aiming to become the oldest world title winner since Ray Reardon secured his sixth success in 1978 at the age of 45, but the 37-year-old hinted a lack of table-time was a concern.
"I was not sure how the match was going to go and not sure how I was going to compete, it was an experiment in some sorts of ways," said O'Sullivan.
"In general I wasn't as slick or as tight as usual. But you've got to expect that if you've not played for a year, you can only get that through matches...it's like trying to get fit for a marathon the day before a marathon starts."