The work ethos of Oldham's new boss Brian Talbot swept them to their second away win of the season.
First half goals by Ernest Cooksey and Paul Murray broke the meanest defence in the Nationwide League on a day City needed a win to go back into an automatic promotion slot.
A delighted Talbot who has seen his new charges take four points out of six said: "I told the players it's amazing what you can achieve when you work hard. We go back on the bus a very happy bunch of players." There was no happiness though in City's dressing room where top scorer Lee Peacock, who missed an early chance, admitted his team never got going.
A blustery wind was always going to make it difficult for ball control and the judgement of passes.
Oldham won because they got the ball down and used it with more conviction. Even though rookie Wes Wilkinson, coming on for only his second match since stepping up from non-league Nantwich Town was often caught off-side - a total count of nine against him - though always bombing in.
Bristol's manager Danny Wilson complained: "We were a shadow of ourselves and were asleep for the first goal." This came when centre back Tony Butler gave away a free kick 35 yards out. Darren Sheridan lobbed the kick over the wall and Cooksey went in unchallenged to place his header well wide of keeper Steve Phillips.
The second goal was even more decisive, Wilkinson sliding a pass forward for Paul Murray to shoot under Phillip's desperate dive.
The goals sprang out of adversity for Oldham who lost midfielder Gareth Owen inside 10 minutes with a damaged hip and Danny Boshell soon followed him with a bang on his right knee.
City were to lose Peacock to an injury but could not seek, and did not seek, that as an excuse.
Oldham's keeper Leslie Pogliacomi had few direct shots to handle, one from Christian Roberts, he diverted over his bar and late on he scooped a Lee Miller header from a free kick in the same direction.