Celtic did well to keep "grinding", according to manager Brendan Rodgers, as Oh Hyeon-gyu's late strike saw the league leaders come from behind to beat a spirited St Mirren.
Conor McMenamin nodded Stephen Robinson's side in front early on, before David Turnbull's fine finish from the edge of the box levelled the scores.
Turnbull missed from the penalty spot just before the break, but with just seven minutes left, Oh fired beyond Zach Hemming to secure all three points and maintain Celtic's advantage at the top of the Scottish Premiership.
St Mirren remain third but, for a time, they might have been within five points of the leaders.
"You have to keep grinding in games like that," Rodgers said. "The winning goal is a real testament to the calmness and quality of the team. In the right moment we speed up the game and Oh scores a great goal.
"In the midst of a busy period it's a really important win."
Robinson had called for his players to raise their already high levels for this game, and they certainly started brightly.
Their early pressure was rewarded when Greg Kiltie whipped in a dangerous ball from the left, McMenamin found space among a static Celtic defence, and the Northern Irishman planted his header past Joe Hart into the corner.
Turnbull returned to the starting line-up in place of the injured Reo Hatate and reminded home fans of his ability when Matt O'Riley created space and found his fellow midfielder, who smashed a wonderful strike into the top corner.
What an impact to ease early frustrations.
However, Turnbull was unable to add a second after a Video Assistant Referee (VAR) review determined that Alex Gogic had handled inside the box.
Turnbull's spot-kick cannoned of Zach Hemming's upright, the goalkeeper then denied Luis Palma on the follow-up and the chance was gone.
James Forrest and Yang Hyun-Jun were both denied by Hemming from close range as Celtic pushed for a winner.
They increasingly dominated the ball and territory but came up against a St Mirren side determined to get something.
They found that vital winner, though, when two substitutes combined.
Odin Thiago Holm played a delightful pass into the path of South Korean striker Oh and he finished emphatically to clinch a vital three points.
Player of match - Oh Hyeon-gyu (Celtic)
Oh takes chance with vital goal - analysis
Celtic came into this after a frustrating draw at Easter Road against Hibernian, perhaps feeling the intensity of their schedule. Rodgers made a few changes that didn't seem to pay off, with recent stand-out O'Riley largely nullified apart from his assist.
Turnbull did make an impact with the equaliser but might have made this a far more straightforward game for his side had he converted the penalty.
Oh did take his chance and finally delivered a goal could prove so important in the long run, for both him and Celtic.
St Mirren are a side bang in form. They came with a clear plan and it so nearly worked. They rode their luck at times but offered an overall performance that showed why they are where they are.
When they fell behind they still mustered a threat despite that blow.
They are a very good side and this experience at Celtic Park will only add to their belief. There's no reason they can't stay in third. None at all.
What they said
Celtic manager Brendan Rodgers: "We made a slow start - just too passive, weren't aggressive enough with or without the ball, and they deservedly went in front.
"After that we sped up the game. They are very well coached so we knew they would have moments, but we kept going. We should have gone in front, we missed the penalty, and that's something as a team we have to be better at.
"Second half we kept going, and the subs come in and make the impact you want."
St Mirren manager Stephen Robinson: "It's a negative in terms of the result, but we wont be taking any negativity for too long. All the good bits we did, we have to keep improving on those.
"There's a big disparity between the clubs but there's not a huge disparity in terms of the playing staff, because when you're organised and disciplined you can show your quality within that.
"Zach Hemming has produced three or four world-class saves and we had opportunities to grab a draw."
Celtic travel to Dingwall for the early kick-off on Saturday against Ross County (12:30 GMT). St Mirren don't play this weekend and host Hibernian next Wednesday (19:45).