O'Sullivan secured a record-breaking 71st victory at the Crucible when sealing a 13-4 win over Mark Allen to move ahead of Stephen Hendry on most matches won at the famous Sheffield venue.
'The Rocket' will now take on Stephen Maguire of Scotland in the last eight of the major after his 13-9 win over Zhao Xintong of China.
O'Sullivan, who led 6-3 and 12-4, needed only 15 minutes on Saturday to take care of Allen and his eye-catching win included two tons and eight 50+ breaks.
The 46-year-old said his sole motivation was to win a seventh World Snooker Championship trophy.
He told BBC One: "For me it is about winning the majors, there is no point coming second because no-one talks about that.
"I have won it six times and it would be nice to win it again, if I don't no big deal as I am still up there with the two or three other guys who are classed as the true greats of the game."
Selby's hopes of back-to-back World Snooker Championship triumphs ended in a 13-10 defeat to Yan Bingtao of China - in a match that included the longest frame in Crucible history.
Selby had trailed 11-7 but closed the gap to 11-10 with breaks of 86, 117 and 88 before a marathon 22nd frame got underway.
It lasted 85 minutes and 22 seconds before Yan emerged on top to go 12-10 ahead and secured the win with a break of 112.
A disappointed Selby told BBC Sport: "Yan is just a great all-round player, you have to beat him, he doesn't give anything to you. You have seen that from start to finish and he plays some very clever shots. I have a lot of respect for him.
"I wish I was as good at his age. For me, he has the game to go on and win it. Here, to not be scoring and not be at your best, you have to scrap it out and he showed that."
World number 16 Yan will now take on Mark Williams on Tuesday and Wednesday.