Bristol City boss Danny Wilson pinned the blame squarely on his captain after going down to a narrow defeat at Boundary Park.
Midfield battler Tommy Doherty was sent off with the score at 0-0 - City's third red card in five away games - and Oldham took advantage through another goal by Clyde Wijnhard.
Wilson fumed: "Tommy was irresponsible and I've had words with him because he let us down.
"We never learn and he made life much more difficult by giving the referee no option.
"The second yellow was possibly innocuous but the first was a reckless tackle and you can't go around doing that.
"I'm sure we would have come away with something but for that and it's important that players use their heads when that little bit of red mist comes down."
Doherty got his second booking after 59 minutes and in-form Oldham bagged the winner 11 minutes later.
Wijnhard, who could be a huge influence in their promotion bid, hit his seventh goal in seven games since signing - and his sixth in the space of a week.
Delighted Latics boss Iain Dowie said: "It might sound odd but I'm more pleased with that result than the 6-1 win against Mansfield on Saturday.
"We didn't pass the ball particularly well but we showed a lot of patience against a very good team."
There were few clear chances in a tight contest between two sides expected to challenge for promotion.
Oldham keeper Les Pogliacomi made the first save from a Brian Tinnion free kick and, after opposite number Steve Phillips denied David Eyres, City again went close through Lee Peacock.
Doherty's dismissal prompted Oldham to go up a gear and they struck after Eyres used his left foot to pick out Wijnhard, who sidefooted home a simple finish from six yards