O’Sullivan missed a black on the 13th frame of their 10-5 first-round victory over David Gilbert and appeared to make the gesture as he returned to his seat.
“We are aware of the incident and it has been referred to the WPBSA Disciplinary Committee,” a World Snooker Tour spokesperson told the PA news agency.
Six-time world champion O’Sullivan, 46, who will meet Northern Ireland Mark Allen then at Le Creuset, faces a possible fine.
Allen criticized the quality of the table after battling his way to a 10-6 first-round success against Scott Donaldson.
Allen was far from his best in a scrappy encounter, but relied on a 5-4 first-session lead to secure his place in the last 16.
“I struggled a bit with the table,” Allen said. Eurosport. “It wasn’t the best I’ve ever played on. In fact, it got worse as the day went on.
“Me and Scott had to talk to the ref to see if it was the same ball game tonight because the table was playing completely different.
” It was hard. It doesn’t lend itself to the finest of snooker when you’re getting big rebounds and you’re scared of when it’s going to happen.
Allen extended their lead to 7-4 in the second session, but a missed red in the 12th frame cost him dearly as Donaldson calmed his nerves to reduce the deficit.
Donaldson won the next frame before the interval, but Allen eventually found form to wrap up the victory with the game’s best snooker.
It doesn’t lend itself to the finest of snooker when you’re getting big bounces and scared of when it’s coming
Mark Allen told Eurosport
Allen said: “To make it 2-2 I felt like I was getting out of jail a bit because he was by far the better player at the time.
“Scott was getting the upper hand, really good safety and I had a lot of clumsy shots. I was just hanging on there.
“I had to find some form because I didn’t think I had much at the start.
“I controlled it from there, but there were still one or two mistakes you can’t afford to make.”
Former world champion Stuart Bingham compiled the biggest break of the tournament by establishing a 6-3 lead overnight against China’s Lyu Haotian.
Bingham, known as one of the great break builders in the game, produced a 140 clearance in the eighth frame.
The 2015 champion lost the first frame after running out of position on a break of 46.
Lyu, ranked 64th in the world, took advantage of Bingham missing a frame-winning blue but quickly trailed 2-1 before hitting back with a 103 break to level.
Bingham took control after the interval to take a four-frame lead before Lyu responded late.
Three-time Crucible champion Mark Williams advanced to the second round with a 10-3 victory over fellow Welshman Michael White – he will now face another compatriot in Jackson Page.
