Jamie Paterson scored two superb second-half goals as Walsall earned their fifth win in six games and ended Paul Dickov's reign at Oldham.
New loan signing Lee Barnard headed the opener for the Latics, who despite beating Liverpool in the FA Cup, have lost eight out of nine league games.
James Baxendale equalised after a pass from Andy Taylor and Fabian Brandy then hit the inside of an Oldham post.
Adam Chambers set up both goals for Paterson, one from 25 yards, to win it.
Dickov left his position as manager less than 24 hours later.
VIEW FROM THE DRESSING ROOM
Paul Dickov, who left his position as Oldham manager after the game, told BBC Radio Manchester:
"In the first 25 minutes, we were excellent. We got in front, looked bright and played some really good football.
"After that, I'm not sure the boys thought it was going to be a little bit easy because we were so much on top.
"They got back in it, we sat off them and that allowed them to get some impetus.
"The desire and intensity we had last week against Liverpool wasn't there."