Substitute Garry Thompson scored a second-half winner for Scunthorpe at Oldham as both sides ended with 10 men.
David Mirfin's opened the scoring for the visitors in the 73rd minute, heading in Josh Walker's corner.
Filipe Morais levelled as his right-wing cross sailed over Sam Slocombe's head and in off the post.
Thompson won the game with a header from Walker's cross and the latter was sent off, along with Oldham's Keanu Marsh-Brown.
Marsh-Brown received a straight red card for his two-footed lunge on Walker, whose angry reaction earned him a second booking.
VIEWS FROM THE DRESSING ROOM
Oldham Athletic manager Paul Dickov told BBC Radio Manchester:
"We're really disappointed in how we lost the game, I felt we thoroughly deserved to win it.
"The first 70 minutes I thought we were excellent, as good as we've been since I've been here.
"We need to take the positives out of the game tonight because I thought there were a lot of them.
"Throughout the team we got a little bit anxious when we got on top, we started forcing things which is what I think cost us."
Scunthorpe United manager Alan Knill told BBC Radio Humberside:
"It was a battling performance, we kind of rode our luck but the good thing is we kept going and kept believing in what we can do and got the two goals.
"We've been on the end of a lot of games like that where we've been the team that's hit the post or hit the bar and had lots of chances and come away losing. It feels good to come away with three points because it was quite a tough game.
"Everytime we put the ball in their box we looked dangerous, especially set plays.
"Anybody who thought it was going to be an easy game was definitely wrong."