Jason Lowe broke Notts County's resistance with an 87th-minute penalty as Oldham snatched a 2-0 win with two late goals at Meadow Lane.
County could have gone behind after 16 minutes when they lost possession and Chris Taylor squared the ball to Aidan White eight yards out, but Stephen Darby's block on the line prevented a goal.
Lee Hughes finally registered the Magpies' first shot on target as he directed a 70th-minute header straight at Dean Brill.
Reuben Reid then fed Kieran Lee, whose cross-shot flew wide with Oumare Tounkara unable to turn it in at the far post.
But with three minutes to go, Mike Edwards ended Taylor's jinking run illegally and after the County skipper saw red, Lowe tucked the penalty low to Stuart Nelson's right.
And with the hosts pouring forward in search of an equaliser, Oldham capitalised as Reid turned his markers one way then the other before thumping home a second.
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