Over two decades on, as O'Neill and company travel to North Lanarkshire once more, the parallels are uncanny. If Celtic lose, and Hearts beat Falkirk at Tynecastle, their title dream is over.
Internally, O'Neill knows how arduous a test Jens Berthel Askou's men will provide. It is why he did not get too ahead of himself after Sunday's victory over Rangers.
With two games to go, the interim Celtic manager refuses to believe his side are title favourites, as he reflected on that fateful afternoon at Fir Park all those years ago.
"It's a different set of circumstances," O'Neill stressed. "It's just ironic that it should be this. It could be all over tomorrow night if we don't get a result. We have to win the game.
"So that still remains a massive disappointment to me, that game in 2005. But a different set of circumstances and this group of players wouldn't even have heard about it.
"I thought we should have won out of the park at the end of that day.
"We missed goal after goal. [Gordon] Marshall was brilliant in goal for them. I remember John Hartson headed one from about two yards, and it hit him straight in the face.
"And you got the feeling as the game was going on, there might be some sort of foreboding there. But listen, it happened. We should have won. We didn't. It played on my mind for about 15 years, but after that I let it go."
Martin O'Neill's Celtic face Motherwell on Wednesday evening.Martin O'Neill's Celtic face Motherwell on Wednesday evening. (Image: Stuart Wallace / Shutterstock)
O'Neill righting the wrongs of one of the most gut-wrenching days of his career 26 years on, and leading Celtic to an unlikely title triumph in the process, feels destined.
But does he believe in fate?
"If it happens, yeah. I'll say that if we get beaten at Motherwell, and that constitutes us losing [the title], then I will never visit Fir Park again in my life. I will take a detour. I'll go to Wishaw instead. I'll visit the Tommy Gemmell statue!"
When Celtic lost 2-0 to Dundee United at Tannadice on March 22, they fell five points behind Hearts, and sat third in the William Hill Premiership table with seven matches remaining.
Fast forward seven weeks, and with two victories in their next two matches against Motherwell and the league leaders, Celtic can become champions of Scotland for a record 56th time.
"We were obviously down after Dundee United and had a reset after that," O'Neill explained.
"Some of the players left for international duty, so I don’t really know what they were thinking.
"But when we came back, there was a mindset that we had to win football matches. Now, saying you have to win football matches and actually doing it are two totally different things. But there was no room for error, as it has been proved.
"We have picked up all our points since then, and we’re still behind. But there was a feeling at that stage where we believed we could still do it."
That said, O'Neill refuses to get too ahead of himself.
"I don’t agree Celtic are now favourites," he stated. "I don’t see it like that. I genuinely don’t see it like that at all.
"Our fixture against Motherwell now is extremely difficult. We have to win the game, and winning at Fir Park is not easy. They have something to play for, too, so it will be difficult.
"I know it seems an easy equation: we just need to win our two games. I would have taken that after we lost to Dundee United, of course. But the two games happen to be extremely difficult."
Given that they have been top of the table since September and remain in pole position ahead of the final two rounds of league fixtures, is there more pressure on Hearts to lift the title than Celtic?
"I don’t know," admitted an honest O'Neill.
"I think you would have to ask them. That sounds like I’m almost trying to pile pressure on someone else. But we have our own pressure we have to deal with."
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But he also outlined the need for a commanding figurehead. The 74-year-old looked abroad to Real Madrid, where Xabi Alonso was dismissed as head coach earlier this season, primarily due to the influence of big stars such as Vinicius Junior, Kylian Mbappe, and Jude Bellingham behind the scenes.
"It would reflect nicely on the team," he said. "I do genuinely think it would reflect on the team to show a bit of character when they could have thought, 'Listen, we’ve just had another manager. How many managers are we in?'.
"We’ve seen the rise of player power and whatever, but I still think they need a boss. I don’t care what you say.
"You have seen numerous occasions recently, Real Madrid, for example, where player power ousted a very talented manager. But deep down, when players leave the training ground, they need someone in charge.
"I suppose, to some extent, I’ve been in charge. Maybe not in the same manner as I was way back."
Celtic are to victories away from winning the Scottish Premiership title. (Image: Stuart Wallace / Shutterstock)
From his initial spell in the Celtic Park dugout over two decades ago, O'Neill admits he has had to adjust to the modern game and all its travails.
"I think I can read players," he said. "I don't think I've lost that. I think I know what players feel during the course of games, and that doesn't change.
"Those sorts of things I can bring. But if you're asking about the last five months or whatever it is, I'm the one who's learning, believe it or not, at 74.
"Various tactical things. There seems to be an overemphasis on playing out now and taking big risks. I sometimes think the risks you take should be in the final third rather than your own. But I am adjusting."
The fate of Celtic's season will be decided in the next 10 days. O'Neill will only reflect on his latest spell in charge of the club with fondness if it ends successfully.
"I actually think I have enjoyed it. It’s a kind of renaissance, something that won’t happen again," he said.
"You’re out of the game, watching on the periphery.
"Suddenly, you are back in again. For all I talk about not enjoying it, people know what I mean. I probably have. I’ve enjoyed working with the players and the coaches; it does give you a lift.
"The next 10 days will determine how you reflect on it. If we don’t win anything, I won’t take any great pleasure out of my time here."
The 2005 defeat to Motherwell ended O'Neill's dream of departing Celtic as a title-winning manager. Lightning couldn't strike twice, could it?