The Saddlers scored a goal in each half through defender Scott Dann and on-loan Oldham striker Michael Ricketts before seeing out their victory with well organised defence.
Oldham went into the match unbeaten in eleven league and cup outings but they were well below par after Dann broke through in only the second minute.
Ricketts, who joined Walsall on loan in November and could play against his parent club as part of the deal, finished them off in style with his seventh goal of the season - two for Oldham and five for Walsall.
The in-form visitors got off to a flying start when young full-back Richard Taundry swung over a free-kick from the left and Dann leapt from eight yards out to direct his header into the bottom corner.
Oldham tried to hit back quickly after Chris Taylor set up Lee Hughes but the finish was well wide.
Hughes then headed over and Gary McDonald did the same while Stefan Stam was off target from a corner.
At the other end Walsall missed a chance through Anthony Gerrard as Oldham's best five minutes of the match failed to bring any reward.
Neal Trotman headed over the bar from close range and Hughes miscued a volley when he looked poised to score from Taylor's cross.
Oldham continued to have long spells of possession but they only tested Walsall keeper Clayton Ince with a shot from Neil Kilkenny.
Walsall doubled their lead on 65 minutes as Ricketts chased down a clearance and burst into the box to slot his shot past keeper Mark Crossley.
Oldham substitute Deane Smalley was unlucky with a diving header but Ricketts went close to adding a third when he hit the woodwork from long range in the 89th minute.