The Cobblers saw boss John Gorman quit on Wednesday insisting he did not need the pressure of management, but there was no respite from their recent poor form.
Oldham, who had been on a superb run until failing to win either of their last two games, bounced back in style.
They were rarely in trouble as Gary McDonald, Chris Porter and Andy Liddell all got on the scoresheet in the first 61 minutes.
Oldham, in front of their biggest crowd of the season, had Porter and midfield man Richie Wellens back from injury and suspension and the pair were instrumental in a fine team performance.
Northampton had only one clear chance in the game, while the fact keeper Mark Bunn was easily their man of the match showed the level of Oldham's dominance.
Bunn made a brilliant save in the first minute as he tipped away a four-yard shot by Oldham striker Paul Warne.
Warne was denied again before Liddell put a good chance over the bar from close range.
The Latics broke through on 22 minutes following a ball by Wellens to left-back Simon Charlton.
Charlton fed Chris Taylor, whose cross was headed into the top corner from ten yards by McDonald.
Bunn made another good save from Porter before Oldham keeper Les Pogliacomi had to superbly turn away an effort by Ryan Gilligan against the run of play.
Oldham doubled their lead just past the half-hour mark as Porter latched on to Warne's flick and brought a great stop from Bunn before following up to convert the rebound past covering defenders from 12 yards.
Northampton had more of the ball after half-time but they could not prevent a third goal for Oldham after Liddell cut inside from the right.
Porter laid the ball off intelligently and Liddell kept his head ten yards out to drill a shot into the bottom corner.
Northampton substitute Jerome Watt had a rare sight of goal but it was Oldham who continued making chances as Porter and Matt Wolfenden both fired wide.