Posh needed only 10 minutes to find the net on Saturday when an Aaron Mclean cross looped in after taking a deflection of defender Stefan Stam.
And it went from bad to worse for Latics when young goalkeeper Greg Fleming dashed out to bring down Mclean just inside the box 10 minutes later.
Fleming escaped with a yellow card and then kept out Mclean's penalty before Posh then missed again from the spot on 37 minutes.
Craig Mackail-Smith took over the penalty taking duties after being sent tumbling by a Reuben Hazell shove, but slammed against an upright. The ball rebounded into the path of Mclean who made no mistake, but Oldham soon came storming back.
Hughes halved the deficit with a shot on the turn five minutes before the break and completed the comeback just before the hour, pouncing after goalkeeper Joe Lewis spilled Andy Liddell's free-kick.