Since the jubilation at Hampden in the League Cup final, it has been a difficult campaign for St Mirren. They won just four league matches in 2026, unable to avoid the play-off spot.
However, when it mattered most, two of the club's senior players delivered two moments of quality to break the deadlock.
O'Hara - introduced from the bench due to a knock - swung a delightful cross towards the far post, and Fraser timed his run to perfection before firing home via the crossbar.
A mention too for goalkeeper Sinclair, signed on an emergency loan from St Johnstone last month after injuries to Shamal George and Ryan Mullen.
He stood up with several saves on a night where fine margins delivered a result of vital importance.
There is plenty of work to do for St Mirren in the off season, in particular whether interim boss McLeish is appointed on a permanent basis, but those conversations will wait until after the celebrations die down.
The ecstasy from most in the ground was in stark contrast to those in the away end, with many in tears as Thistle supporters endured a familiar sinking feeling.
They were the better side in the first half, but failed to convert some tidy passing play into clear sights of goal.
Aidan Fitzpatrick had a shot deflected wide, while Robbie Crawford headed wide before shooting at Sinclair.
It was a more even contest in the second period, but where Fraser found the back of the net, no-one in red and yellow was able to do so.
There were huge shouts for a penalty in second-half stoppage time as Gary Mackay-Steven went to ground, but replays showed defender Miguel Freckleton got the ball.
Thistle did force a free-kick on the edge of the box for the final kick of the game, but Kyle Turner's effort went straight into the wall.