O'Sullivan claimed five of the eight frames in the penultimate session of his world championship semi-final against Scotsman and fellow veteran John Higgins.
The 46-year-old was 10-6 ahead at the start of Saturday's early Crucible session and put on a high-quality display to seal a 15-9 advantage.
O'Sullivan was clinical as he produced three centuries and took advantage of every Higgins slip up.
The Scot displayed his frustrations as he slammed his cue into the floor after letting O'Sullivan in during the 22nd frame.
Higgins did take the opening frame of the morning session, but failed to back it up as O'Sullivan reinstated his four-frame advantage with a break of 82.
The Scot, who is a four-time world champion, came back fighting, but his impressive ton was swiftly followed by one from his opponent.
Higgins fluffed an easy red to the middle pocket in the following frame and opened the door for O'Sullivan, who fired a devastating 121 - the 100th century of the tournament.
The six-time world champion got away with an error in the next frame and went on to seal a 15-9 lead going into this evening's session.
O'Sullivan needs just two more frames to secure his place in the final. The snooker great is looking to emulate Stephen Hendry by winning a seventh world crown.