Oldham preserved their season long unbeaten away record and left Mansfield deep in the relegation mire thanks to a controversial late penalty in a match which saw three players sent off and eight booked.
Athletic striker Wayne Andrews, who was lucky to still be on the pitch after an off the ball clash with Adam Eton in the first half, won the vital 87th minute spot kick after being tugged back by Liam Lawrence.
Fitz Hall converted the spot-kick to clinch Oldham's second win in six games and revitalise their hopes of automatic promotion.
It was rough luck on relegation-threatened Mansfield who dominated the first half and then three times went close to a second-half breakthrough.
Oldham had been reduced to ten men in the 34th minute when David Eyres was harshly adjudged to have kicked Craig Disleay off the ball.
But in the 54th minute the numbers were even when Rhys Day was also ordered off for a foul and then deep into injury time Mansfield player-manager Keith Curle was red-carded.
Just four Stags players remained from the side that had been hammered 6-1 at Oldham in September, with a totally new defence lining up against the surprise returning Clyde Wijnhard, who had struck four goals in that drubbing.
Wijnhard, out for 13 games with a serious knee injury, almost made an immediate impact.
In the third minute, he sprung the offside trap, raced on to Andrews' flick-on but fired over with Oldham's only shot of the first half.
Mansfield's reply was instant, winger Wayne Corden skipping past two defenders on the edge of the area before curling a 20-yard drive narrowly wide.
In the 17th minute Athletic were given a double let-off. Defender David Beharall deflected Lawrence's cross goalwards forcing keeper Les Pogliacomi into a quick reaction save.
From the resulting corner Iyseden Christie somehow drove wide from just six yards.
In the 27th minute Junior Mendes also went close, lifting Christie's cross over the bar from close range.
A minute into the second half substitute Lee Williamson's piledriver was tipped away for a corner by the flying keeper, who later did well to twice deny Lawrence from 20 yards.