Lee Grant dived brilliantly to his right to save Alex Russell's 55th minute kick after Guy Branston had handled. But the referee ordered a retake because of encroachment and this time Russell thumped it straight as Grant again dived the same way.
Oldham were incensed but then luck was with them as David Noble slammed a free kick against their bar and the home side had a second penalty appeal turned down, this time for hands against David Eyres.
Bristol had never come from behind this season to win but they did so this time when Branston gave away a rather unnecessary corner. The kick was taken by Craig Woodman and was flicked on to Stephen Brooker to claim his 15th goal with a firm header at the far post.
Returning from injury Brooker had entered the game early as the sub when Aussie Luke Wilkshire had to go off with a cut mouth.
Too much of the early play had been leggy reflecting a second game in three days but when the breakthrough came for Oldham it was from a stoppage time corner. City had been in trouble from an earlier corner and Eyres, one of four changes made by Ronnie Moore, tested them again and found them wanting. His deep kick beyond the far post saw Luke Beckett out jump defenders to place a 17th of the season beyond Brazilian keeper Adriano Basso.
They had already had two tense duals Basso denying the striker midway through the half with a save and then wrong footing him as Beckett raced onto a chip from Richie Wellens into the six yard area.
City had their best chances from runs by David Cotterill and Scott Brown and a Noble free kick which was homing in under the bar.