The impact was immediate, the Derryman proceeding to fillet his Welsh striker before sending him to Coventry — figuratively, that is — for five days before the thaw came.
It was Hartson — who once kicked Eyal Berkovic in the head when the players were at West Ham — who climbed down first but once he did O’Neill accepted it, erased the incident from the charge sheet and the pair picked up as before.
It is just one of a plethora of stories about the new Republic of Ireland manager that littered Alex Montgomery’s 2007 biography of him, and Hartson was the source of a fascinating wider treatise on his then manager.
The perception that O’Neill somehow ruled by fear was shredded.
Though he had been on the receiving end of Brian Clough’s withering tongue more than most at Nottingham Forest, he has tended not to ape such caustic extremes as a manager himself.
Hartson insisted the majority of players who featured under O’Neill at Parkhead would have gladly followed him south of the border when he pitched up at Aston Villa, but that isn’t to say they never felt his ire.
With O’Neill — like Alex Ferguson, whom he met at Dublin Airport on Saturday — the secret to using a hair dryer is not so much how you use it but rather how often.
Call it the carrot and the stick or the iron fist inside the velvet glove, there was a fascinating glimpse of that duality as he came towards the end of his media duties in Dublin at the weekend. The endless questions about Roy Keane had barely ruffled a feather.
Queries about Denis O’Brien’s contribution to his salary were batted away and remarks about the reaction to his appointment from north of the border summarily dismissed.
For an hour-and-a-half O’Neill dealt breezily with every potential curveball, but then someone went and mentioned Paolo Di Canio.
The same Paulo Di Canio, that is, who succeeded him as manager at Sunderland and proceeded to dump all over his work by way of criticising everything from the condition in which he left the first team to the condiments on their tables.
“Paolo Di Canio?,” he said when asked if the Italian’s criticisms had hurt. “That managerial charlatan. Absolutely, yeah. Paolo stepped in there and as weeks ran on, he ran out of excuses. I had a wry smile to myself…”
Maybe at the time but any hint of a grin had long since evaporated by the time he sat down with the Irish press and Di Canio’s willingness to trace all of Sunderland’s ills directly back to his door clearly left their mark.
“It’s like a 27-year-old manager stepping in and the first thing you do is criticise the fitness of the team beforehand. If you’ve ever seen Aston Villa play, you’ll see the one thing I pride myself on is teams being fit.
“What you’ll find interesting is that when he started, the team wasn’t fit for the Chelsea game. Then the following week when he won at Newcastle, not being fit wasn’t mentioned.”
It’s not hard to imagine O’Neill sitting around the Ireland team hotel with John O’Shea, Keiren Westwood and James McClean at some stage this week and trading stories about the volatile Italian’s brief time in charge.
Prominent in their discussions, no doubt, will be the proclamations banning everything from mobile phones, ice in their Coke, ketchup and mayonnaise and O’Neill was keen to lay that one on thick.
“I’m hoping at some stage or another John O’Shea asks me at the dinner table to pass him the tomato sauce and I will dispose of it immediately. But then if I feel you can’t win games without tomato sauce I will empty it on his plate, with the chips.
“If somebody could explain to me, Paolo, who is Italian … John Robertson once said that if every team in Italy has pre-match pasta for their meals, how come three get relegated each year? It’s an interesting point. Ability might come into it.”
O’Neill ended with one more dig, a wistful remark about how he would have liked the opportunity to sign 15 players as Di Canio had, but then the darkness passed and it was smiles and affability all round again.
The bad cop’s business was done.
For now, anyhow.