O'Sullivan was forced to declare on 104 before he won a scrappy second frame to move 2-0 clear. A knock of 67 saw Pinches close to 2-1 behind, but further runs of 50, 44 and 58 secured O'Sullivan's passage to the last 32.
"I know what a class player Barry is, he beat me in the final of a PTC tournament a couple of years ago," said O'Sullivan, winner of the Welsh Open in 2004 and 2005.
"He had his chances tonight but didn't play as well as he can so I am just pleased to get a win. I would have liked to have made the 147 for the fans. it would have been good to take me to 12 maximums to the top of the list (he shares the record of 11 with Stephen Hendry). I love this venue and the crowd here - it would have been nice to get it for them.
"Xiao is a good player, I have practised with him a lot. He is breaking through to the top 16 now and it will be a tough match for me. Last week I wasn't sharp because I have hardly picked up a cue and only played a few hours to prepare. It was a struggle but I have played a bit since my last game and I just need to keep going. I have played about as half as much as everyone else so it is more important for me to get results and get momentum on my side. It will be better if I play more but I have decided I don't want to do that, so I have to do my best and see what happens."
Neil Robertson also completed a comfortable 4-0 victory over Rory McLeod to join O'Sullivan in the last 32. O'Sullivan will meet Xiao Guodong for a place in the last 16.
Robertson ran in 104, his 87th century of the season to secure a match with fellow former world champion Mark Williams, who enjoyed a 4-0 win over Tian Pengfei.
"I had a good few days off after my first round match and picked up where I left off," said Robertson. "Overall I played really solid, Rory is a very tough player and it wasn't going to suit him to make it an open affair. I won a couple of tight frames and I am really happy with my performance.
"The plant that I knocked in during my century was absolutely crazy. If I have a good run at the World Championships then who knows, I could get tto 100. But I don't want to start sacrificing frames to chase the record. It is already a fantastic record, every single one that I make will be a bonus. But it would be unbelievable to get 100 in a season. The perfect situation would be to be on 99 going into the World Championship and to complete the 100 at the Crucible.
"Mark and I haven't played each other too much over the years, he will be keen to do well in front of a big crowd. But I think the crowd will be fair, I always get good support wherever I go even though there will be more for him as he is on home turf. I don't think it will be too long before he turns it around, I am sure if he works hard he will get his game back."
Shaun Murphy made runs of 66 and 55 in his 4-2 win over Craig Steadman, Barry Hawkins edged Cao Yupeng 4-3 with Jack Lisowski running in 67, 64 and 81 in his 4-1 win over Martin Gould.
2009 Welsh Open champion Ali Carter overcame Mike Dunn 4-3 with top runs of 62 and 86 while Ben Woollaston completed a 4-1 defeat of Alfie Burden. Glasgow's Anthony McGill overpower Welshman Ryan Day courtesy of runs of 63, 90, 65 and 84.
Tuesday 25 February
Second round (last 64, best of seven frames)
Graeme Dott (Sco) 4-3 Jamie Jones (Wal)
Fergal O'Brien (Ire) 0-4 Liang Wenbo (Chn)
Jamie Cope (Eng) 1-4 Ding Junhui (Chn)
Anthony Hamilton (Eng) 4-1 Li Hang (Chn)
Ben Woollaston (Eng) 4-1 Alfie Burden (Eng)
Mike Dunn (Eng) 3-4 Allister Carter (Eng)
Tian Pengfei (Chn) 0-4 Mark Williams (Wal)
Anthony McGill (Sco) 4-1 Ryan Day (Wal)
Neil Robertson (Aus) 4-0 Rory McLeod (Eng)
Shaun Murphy (Eng) 4-2 Craig Steadman (Eng)
Martin Gould (Eng) 1-4 Jack Lisowski (Eng)
Cao Yupeng (Chn) 3-4 Barry Hawkins (Eng)
Stuart Bingham (Eng) 4-2 Thepchaiya Un-Nooh (Tha)
Barry Pinches (Eng) 1-4 Ronnie O'Sullivan (Eng)
Mark King (Eng) 4-2 Alan McManus (Sco)
Ricky Walden (Eng) 4-2 Matthew Selt (Eng)
Wednesday 26 February
Third round (last 32, best of seven frames)
Shaun Murphy (Eng) v Liang Wenbo (Chn)
James Wattana (Tha) v Joel Walker (Eng)
Scott Donaldson (Sco) v Andrew Higginson (Eng)
Dominic Dale (Wal) v Mark Selby (Eng)
Ricky Walden (Eng) / Matthew Selt (Eng) v Anthony Hamilton (Eng)
Matthew Stevens (Wal) v Joe Perry (Eng)
Mark Allen (NI) v Allister Carter (Eng)
Neil Robertson (Aus) v Mark Williams (Wal)
Stuart Bingham (Eng) / Thepchaiya Un-Nooh (Tha) v Michael Holt (Eng)
Xiao Guodong (Chn) v Ronnie O'Sullivan (Eng)
Graeme Dott (Sco) v Marco Fu (HK)
Judd Trump (Eng) v Anthony McGill (Sco)
Jack Lisowski (Eng) v Barry Hawkins (Eng)
Mark King (Eng) / Alan McManus (Sco) v John Higgins (Sco)
Stephen Maguire (Sco) v Andrew Pagett (Wal)
Ben Woollaston (Eng) v Ding Junhui (Chn)