The quick-fire snooker event is a best-of-three frame competition in the early stages so participants have seven mini matches to try and progress from their group.
The top two from each group go through to the knockout stages but O’Sullivan will not be joining many of his peers.
The 45-year-old got off to a decent start as he beat Jamie Wilson 2-0.
However, he then suffered a shock 2-1 defeat to World No 120 Ben Hancorn, despite making a superb 141 break.
The six-time world champion turned up late for his match so had to start the tie 1-0 behind.
That seemed to fire up the Rocket and he won back-to-back frames in just 14 minutes to notch his second win.
Lu Haotian beat O’Sullivan in the following game to confirm that the snooker legend could not make it to the knockout stage.
On Sunday Mark Selby detailed how strange he found the WST Pro Series.
“Best of three is cut-throat, anybody can go through,” said Selby.
“To finish top of the group is pleasing. More pleasing that I felt like I played well today. I scored ok when I got my chances, I had a couple of 140 breaks. That was the main thing for me, to find a bit of form.
“It’s the same for everyone and it is nice to get through to the next group.”