After his near-flawless 4-0 victory over John Higgins, O'Sullivan and his opponent Judd Trump both made nervy starts, missing a number of simple pots.
However, O'Sullivan eventually went 1-0 up in the 29th meeting between the pair, before Trump levelled after yet another error-strewn frame.
'The Rocket' finally found some momentum in the third frame, coming up with a brilliant break of 123. They continued to seriously struggle though, before O'Sullivan finally took a 3-1 lead after two fortunate pots and a series of uncharacteristic mistakes from Trump.
He then wrapped up a 4-1 victory following some more Trump errors, and will now be targeting his fifth title of the year later tonight.
"It was weird. It wasn't great. Judd was probably the worst I've ever seen him play. I think I dragged him down to my level," the seven-time world champion said.
Trump was equally down about his performance.
"I didn't play very well. I felt terrible, I felt shaky. One of the worst performances I've had in a long time. I didn't feel good at all," he said.
"It's been a good season so far but that was one of the worst I've felt for a while."
The other semi-final earlier on in the day was a hugely exciting and high-quality affair. Brecel got off to a great start, racking up two 50+ breaks to take a 2-0 lead. But Mark Allen fought back, producing a stunning 121 break to half the deficit.
The centuries continued to flow as both players produced almost faultless snooker, with Brecel making a 125 break before Allen's 133.
Brecel, who has struggled for his best form over the last few months, sealed a 4-2 victory with another impressive break of 82 to reach his first final since September.
"Played good, felt good. Every week I seem to play a little bit better, starting in Wales, now I feel very good in myself," the world No.4 said.
Allen was also full of praise for his opponent, saying that Brecel "looked like the Luca of last year".
O'Sullivan won his last meeting with Brecel 11-9 in the Shanghai Masters final, while Brecel reigned victorious in the quarter-finals of the World Championships last year on his way to the title.
