The feelgood factor is well and truly back at Celtic Park.
After Nancy was sacked earlier this week, the Celtic support needed something to cheer about. They got it when O'Neill was confirmed as interim manager again.
While there were perhaps questions about whether the 73-year-old could replicate the form of his previous interim spell - during which he won all of his Premiership fixtures and seven out of eight games overall - any doubt was cast aside here.
It was as emphatic a victory as you can get - a clean sheet, four goals, 22 shots (eight on target), 78% possession, and most importantly three points.
Celtic looked like they were playing with a confidence and self-assuredness that was sorely lacking during Nancy's tenure.
They were solid at the back - Liam Scales in particular looked back to his best - and looked dangerous in attack, and were unlucky not to have had a few more goals.
For United and Jim Goodwin the worries continue.
They failed to attempt a single shot on target, and have now gone 51 games since last winning at Parkhead in December 1992.
This was the first time since facing Aberdeen in October 2024 that they have not managed an attempt on target in the Premiership. And their three shots is the lowest in a league game this season.
Their 22% share of possession speaks volumes about the lack of control they had.
The table is starting to look a little ominous for Goodwin. They stay level on points with Aberdeen and only three above Dundee - both of whom play on Sunday - and drop five behind Falkirk having played a game more.