Just for good measure Rocket Ronnie included a 144 total clearance in a 5-2 victory that booked a meeting with local favourite Mark Williams.
"I was a bit edgy all through the game and wasn't in the comfort zone," O'Sullivan said. "A 147 is just one frame. I'd tell you when I was playing well - I wouldn't lie."
O'Sullivan realised his chance was on for a 147 as early as the first four balls of the break. "I had to knock in a good red on 48 and after that all I had to do was hold myself together."
Despite the two big breaks, O'Sullivan was still tied at 2-2 at the interval. "I just had to concentrate on winning the match after that because that was the most important thing."
O'Sullivan's maximum was the second of his professional career, following his 320-second 147 against Mick Price at the 1997 world championship.
This latest table clearance lasted six minutes and 51 seconds but still brought the house down at the Cardiff International Arena.
It was a superb potting demonstration by the 23-year-old Essex cueman, who just two months ago withdrew from the UK Championship for health reasons.
"To make a break like that he must have so much talent," said Thai-born Wattana, who has compiled three maximums in major competitions himself. "Ronnie said he was shaking as he came to the last few balls, but it didn't look like it to me."Reuse content