Latics striker Charlie MacDonald was twice denied by the woodwork as the visitors went down to their fourth defeat in five matches.
The game begun disastrously for Oldham with Lee Johnson's men conceding the first goal for the eighth time in nine League One matches after only six minutes.
Defender Walsh struck for Crawley from close range when Nicky Adams played the ball back in after the Latics failed to clear a corner.
Oldham had a clear opportunity to hit back midway through the first half when former Crawley striker MacDonald raced through but he was denied by goalkeeper Paul Jones in a one-on-one situation.
The visitors threatened again through MacDonald, who headed onto the outside of the post following a cross from the left by Joseph Mills.
Crawley striker Emile Sinclair should have hit the target just before the break when getting clean through, but he wastefully put an angled shot into the side netting.
MacDonald was unfortunate again shortly after the interval when he headed onto the crossbar after keeper Jones failed to claim a cross cleanly.
Irish striker Billy Clarke should have done better for Crawley after 71 minutes when he put an Adams pass wide from a good position.