Goalkeeper Dean Delaney took the blame as Port Vale were felled by an 87th minute decider at Boundary park.
A David Eyres cross which had missed everybody in the penalty area somehow found its way inside Delaney's far post to leave manager Brian Horton fuming.
Horton said: "It might have been a freak goal, but it was also a bad one to give away and I have to say Dean was at fault.
"He has had a good season, but that goal has gone in from 40 yards without anyone touching the ball and I am afraid that is down to the goalkeeper." Delaney had played well up to that point with his saves keeping Vale in the hunt against in-form Oldham.
The Latics are now unbeaten in nine matches and boss Iain Dowie is delighted with their steady climb up the second division table.
Dowie said: "It was a good game of football which we shaded in the first half even though our passing wasn't as slick as usual.
"Our second-half performance was much better and at times some of our football was really excellent." Vale could have gone in front after four minutes but teenage striker Billy Paynter fired wide from sixyards after getting behind the home defence.
Chris Killen saw his header clawed out by Delaney before the keeper again thwarted former Vale loan player Chris Killen, by kicking away a low shot.
Steve McPhee might have done better at the other end, but Oldham had the edge and were unlucky when Killen's shot took a deflection and clipped the crossbar.
Dowie's men finally got a breakthrough after 71 minutes as Calvin Zola headed a corner against the woodwork and Killen produced an overhead kick from six yards for his second goal this season.
Vale levelled seven minutes later as McPhee was chopped down right on the 18-yard line by Mark Hudson and Mark Bridge-Wilkinson put his penalty in the bottom right corner as Les Pogliacomi dived the other way.
Pogliacomi saved well from Paynter as the visitors pressed on, but Oldham sealed it when Eyres took a free-kick near the right touchline and watched it fly all the way in.