A quickfire double from defender Aaron Brown and former Latics striker Paul Warne crowned the visitors' accomplished display against a home team who are showing inconsistent form.
Oldham pulled off a superb 3-0 victory over League One leaders Scunthorpe in midweek, but they were well below their best just a few days later.
Yeovil took full advantage as they kept a third straight clean sheet and extended their unbeaten run to four games.
Glovers goalkeeper Asmir Begovic was making his last appearance on loan from Portsmouth and defied everything Oldham could throw at him.
Brown also excelled at the heart of defence, where another former Boundary Park player, Gareth Owen, came back to haunt his old club.
Begovic was in the action after just three minutes as he plunged to his left to make a fine reaction save after Lee Hughes got the final touch six yards out.
He also thwarted Lewis Alessandra when the teenage striker, scorer of a hat-trick against Scunthorpe, shot from the edge of the box.
Yeovil were unlucky when Gary Roberts took aim from 25 yards and saw his effort flick the top of the bar.
Oldham, however, had most of the possession and Andy Liddell could have broken through just after half-time.
Begovic saved from Danny Whitaker but Yeovil went in front just after the hour when Roberts sent in a free-kick and Brown guided it home with a firm downward header from eight yards.
Soon they were two up as Oldham's defence was caught at sixes and sevens, allowing Warne to steal behind them and force his shot past Mark Crossley from close range.
Oldham rarely threatened a comeback and Begovic had the final word with a flying save from Hughes.