As O'Neill throws himself into planning for the new campaign, he has had little time to digest last season's success.
Celtic secured their 14th Premiership crown in 15 years in remarkable fashion, winning their final seven matches to snatch a title that had at times seemed to be edging out of reach.
"I feel as if, no it hasn't [sunk in]," O'Neill said.
"If I'd no longer been at the football club, I think I would have more time to digest it and say, oh that was really great, surreal.
"It feels as if you're just back in again, and so I probably haven't got the time. Maybe the first league game at Celtic Park, maybe I'll think about it then, but genuinely, no I don't think it has really sunk in."
The gruelling, draining nature of the job and title triumph left O'Neill unsure if he was open to taking the post full-time this summer. But the potential for regrets pushed him back into the dugout.
"It was strange in the sense that if I'd been asked about it a day after we'd done it, I don't think I would have had the energy," he said.
"And I suppose deep down really, if I felt okay by maybe late July, August or September time and things had moved on without me, then you think, oh gosh, maybe I would like to have stayed on.
"So I tried to project myself a couple of months further forward."
The Champions League incentive also played a part. Celtic's play-off tie is looming large - the first leg takes place on 18 or 19 August - and O'Neill is well versed in the competition's importance to the club.
Celtic, then under Brendan Rodgers, suffered a calamitous qualifying exit to Kazakhstan's Kairat Almaty last summer to set the tone for their turbulent season.
"Absolutely vital," O'Neill said of reaching the league phase.
"If we're going to do it, first of all, we're talking about trying to augment the squad. Secondly, it's getting players prepared both physically and mentally for those games, and that becomes very, very important.
"The disappointment of last season, not making it into the Champions League because of having a good run the previous year, I think that carried over to league performances. Malaise is far too strong a word but disappointment and concern about everything and as a consequence I think we fell down a little bit but we raised it at the end.
"Now this tie that we will be in, whoever it may be, becomes very important."
