Mansfield briefly threatened a comeback in the second half, but there was rarely much doubt about which side would reach the Carling Cup second round.
Summer signings Neil Kilkenny and Jean-Paul Kalala both got on the scoresheet, youngster Deane Smalley bagged his first senior goal and Craig Davies made it two in two games so far this season.
It all added up to an unhappy return for Mansfield boss Billy Dearden - Oldham born and bred and a Latics players back in the 1960s.
Mansfield striker Michael Boulding went close with an early effort, but Oldham broke through on 20 minutes after the visitors failed to deal with a corner.
Kilkenny seized on the loose ball 12 yards out and drilled his low shot past veteran keeper Carl Muggleton.
It was soon 2-0 after Davies got away down the right wing and delivered a cross which Muggleton was unable to snatch at his near post.
Kalala was first to react, drilling in from a couple of yards for his first goal since joining Oldham from Yeovil Town.
Oldham took total control and Muggleton denied Davies and former England striker Michael Ricketts headed wide from close range.
Mansfield created virtually nothing yet unexpectedly they grabbed a lifeline on 72 minutes when poor defending at a set piece allowed defender John Mullins to head home from close range.
Oldham restored their two-goal advantage eight minutes from time when the pace of Davies again caused problems and he centred for Smalley to tap in and make the game safe.
Davies was still not finished, crowning his man-of-the-match performance as he left the Mansfield defence trailing in his wake and fired into the bottom corner from 12 yards.