Welsh wizard Gavin Williams lit up Huish Park with a first-half brace to inspire the Glovers to their first league win in four games.
The visitors took the lead with their first attack when Reece Wabara's teasing cross was headed home by Jose Baxter.
Yeovil striker Paddy Madden thought he had brought his side level after 28 minutes, only to see his header ruled out for offside.
The Somerset side did hit back 10 minutes later courtesy of a driven Williams penalty after Ed Upson was chopped down by Jean-Yves Mvoto.
Williams grabbed a spectacular second before the break, hitting an unstoppable, swerving 25-yarder past the helpless Dean Bouzanis.
The Latics pushed for an equaliser in the second half with Wabara and Baxter both going close, before James Hayter coolly converted a third to give the hosts a two-goal cushion.
Oldham's misery was compounded when substitute Robbie Simpson was sent off for a challenge on goalkeeper Marek Stech, with Madden then adding a fourth goal to complete a miserable day for Paul Dickov's side.
