Jonathan Grounds' bizarre goal helped Oldham beat Portsmouth in a fiery match which saw both end with 10 men.
Liam Walker and Izale McLeod had early chances for the hosts but they were denied by Latics keeper Alex Cisak.
And they were made to pay when Grounds' cross from the left evaded everyone, including keeper Mikkel Andersen, and went into the net.
Pompey's Jonathan Obita and Oldham's Cliff Byrne saw red for violent conduct but it did not effect the score.
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Portsmouth manager Michael Appleton told to BBC Radio Solent:
"We just got sucked into giving cheap free kicks away and letting long balls and long free kicks get played into our box time and time again.
"The referee lost the plot. The game was probably a little bit too big for him and maybe the occasion was too big for him in terms of Fratton Park.
"I think the game just got out of his hands. There wasn't a serious tackle in the whole game."