Oldham had the perfect chance to take the lead after just six minutes when Dale Stephens was fouled in the box but Paul Heffernan’s penalty was saved by goalkeeper Scott Shearer.
The Latics were reeling three minutes later when John Akinde pounced on a loose ball after Darryl Flahavan saved from Kevin Betsy.
But Akinde turned villain after 22 minutes when he was sent off following a raised arm against Oldham captain Sean Gregan.
Oldham took advantage of Akinde’s stupidity on the stroke of half-time when Danny Whitaker’s 30-yard scorcher flew into the bottom corner.
Reuben Hazell headed home Stephens’ corner six minutes after the restart to put Oldham in front, but Wycombe were level when Scott Davies was brought down by Whitaker and Harrold held his nerve.