The Wales striker ended a four-match goal drought with a treble to send the champions back to the top of the Scottish Premier League with a 4-0 win at Almondvale.
Hartson now has 27 goals for the campaign his best-ever season total and with Rangers being left to rue a glut of missed chances against Dundee United on Tuesday night, O'Neill believes he could have hit the goal trail at just the right time.
The Celtic boss said: "John was a bit concerned about not scoring because it was the longest spell he had gone not scoring here.
"I knew he would score at some stage or another but he has rewarded us tonight, so that was delightful.
"The goals and his second-half display will give him an enormous lift. John will be lifted because scoring is what he does best.
"He is the leading goalscorer in the SPL and he hasn't even taken penalties all season. We have been searching for goals and that will give him a big boost."
Livingston manager Richard Gough was impressed by Celtic's edge.
"I thought 4-0 tonight was a bit harsh," said Gough. "But that's what happens when you play a team with the quality of Celtic.
"They finished everything and capitalised on our mistakes, so there lies the difference."
Celtic had to pounce on Rangers' slip-up, which pleased O'Neill, whose side also moved to within just one goal of Alex McLeish's men.
"Rangers' result was unexpected but you have to try and do something about it," he added.
"Our concentration was absolutely vital so it was tremendous to get the points and goals."
O'Neill is sweating on the fitness of Chris Sutton and Bobo Balde ahead of the weekend's game with Aberdeen at Celtic Park after hobbling off.
He said: "That would be a concern at this minute. Chris just went over on his foot and was feeling a wee bit stiff at half-time but we still needed him out there. Bobo has got a kick and David Marshall somehow connected with him. It's all down the front from his knee to his ankle. It's the length of time to recover that's the problem."
LIVINGSTON: McKenzie, McNamee, Rubio (McPake 83), Dorado, Bahoken (Nouma 68), Dair, Deloumeaux, Vincze, O'Brien, Lilley, Kachloul.
CELTIC: Marshall, Valgaeren, Balde (McGeady 88), Varga, McNamara, Petrov, Sutton (Henchoz 77), Lennon, Thompson, Bellamy, Hartson.
Referee: M McCurry.
Scottish Premier: Hearts 1 Hibernian 2
Coca-Cola Football League One: Swindon 0 Bristol City 0.