McGill took a 2-0 lead, thanks to a 116 break in the opening frame and a shock looked on the cards.
However, O’Sullivan took control, firing five half-centuries on his way to winning six frames in a row.
O’Sullivan, who won his first title in Preston 30 years ago to the day, told the BBC: “You just get to the point where you think, ‘I’ll just keep going as long as I can’.
“I was at 35 and I was probably done and dusted because that is what happens.
“But I just appreciate every moment now, it feels like I have got a second life. I am a born-again snooker player.”
“I do find the matches hard and long now. If I can win two or three matches in the tournament, you get used to it. I find it harder to concentrate and have the hunger.
“For me, playing in competitions, you have to dig deep and out-battling players, it is hard at my age.
“These tournaments create a bit of a buzz.”
John Higgins also advanced through – beating Joe O’Connor 6-3 – with the highlight of his performance coming with an eighth-frame 122 clearance.
Neil Robertson suffered another early exit as he was outclassed by Zhou Yuelong in the evening session.
The Chinese hit breaks of 136, 134 and 99 on his way to a 6-2 victory, which saw Robertson lose early in a seventh ranking event so far this season.
O’Sullivan will meet Robert Milkins in the next round after his compatriot claimed a final-set decider against Thepchaiya Un-Nooh.
Milkins had looked dead and buried when his Thai opponent led 5-3, but a 121 break kick-started a run of three successive frames to claim victory.
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