After taking just one point from six, John Sheridan's battlers put their promotion hopes firmly back on track.
They always had too much in the tank for a Northampton side, who had won their last three games at home.
A delighted Sheridan said: "That was a massive win. We have come back before after a defeat and the lads again showed the character that was needed."
Oldham made the best possible start with a quickfire strike after just 61 seconds.
Richie Wellens, perfectly placed 20-yards out, picked up a loose clearance and let fly for his third goal of the season.
Northampton, thrashed 3-0 at Boundary Park just before Christmas, hit back to equalise after 24 minutes.
The Latics failed to defend a free kick and Andy Kirk cleverly placed an angled shot past keeper Les Pogliacomi.
But Oldham, the division's top scorers were back in front two minutes before the break when a great run from Chris Porter set up Paul Warne for his ninth goal.
Leading marksman Porter took his tally to 19 when he chased a long ball from Will Haining.
The Northampton keeper came off his line to try to shut him down but Porter won the race and headed in from 20-yards to end a five-match barren spell.
Not for the first time though, Oldham made it a nervous finish when substitute Jordan Robinson cut the gap to 3-2 - but they held out for the final five minutes.
Sheridan added: "We had to hang on at the end, but we deserved all three points."
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