First-half goals by top-scorer Chris Porter and Andy Liddell sentenced the home side to a fifth straight defeat.
But they are safe from the threat of relegation, thanks to Bradford losing at Chesterfield.
Oldham started much the brighter. Chris Taylor went close with a deflected volley and Porter with an overhead kick.
Paul Warne also spurned two good chances, completely missing his kick from a cross by Liddell and then seeing another effort blocked by Kerry Mayo.
They went in front in the 18th minute when Porter rose majestically to head in a cross from Neal Eardley for his 22nd goal of the season.
Liddell doubled the Latics' lead in stunning style in the 40th minute after both Will Haining for the visitors and Alex Revell for Brighton had hit the bar with headers.
Set up by Warne, Liddell rifled a right-foot drive into the top corner from 20 yards.
Oldham were quite content to maintain their cushion in the second half and Brighton rarely looked like breaking through a defence strongly marshalled by the experienced Sean Gregan.
Les Pogliacomi, Oldham's Australian goalkeeper, had little to do apart from routine saves from a Gary Hart shot and another from Dean Cox from outside the box.
Pogliacomi had no chance five minutes from the end when Cox dipped a spectacular shot over him from outside the area for his tenth goal of the season, but it came too late to save Brighton.