Both players resumed their matches with commanding 12-4 leads, facing China's Pang Junxu and Ding Junhui, respectively.
Brecel, the defending champion, was first to wrap up his match, producing a composed break of 71 to claim the 17th frame 76-0 and complete a 13-4 win over Ding, the 2016 runner-up.
Luca Brecel, speaking to the BBC, said he was looking forward to some potential fireworks during his forthcoming quarter-final clash with Judd Trump: "It could be (explosive) but it could also go scrappy and you never know how the balls go.
"But if we both play well, it should be a cracking game."
O'Sullivan, chasing an unprecedented eighth Crucible crown, swiftly followed.
Although Pang opened their frame with a 17-point contribution, O'Sullivan responded with a clinical break of 95 to match Brecel’s 13-4 scoreline.
"This will be the acid test because he does score," O'Sullivan told the BBC afterwards.
"The opponents I've played in the first two rounds have had enough chances to put me away but they haven't.
"I think this guy will - he beat me in Saudi Arabia this year, I was scrapping but his scoring power was just too much for me.
"Three-session game, best of 25, I'm going to have to see where I'm at."
The quarter-finals will now see Belgium’s Brecel take on 2019 champion Judd Trump, while O'Sullivan will face Si Jiahui, the young Chinese star who reached the semi-finals two years ago.
With this latest win, O'Sullivan extends his own record by reaching the Crucible quarter-finals for a remarkable 23rd time.