The Cod Army's first away defeat since November left them seven points adrift of second-placed Bolton with only four games to play, but the result was also a huge lift for Oldham's survival hopes.
Oldham's Lee Erwin missed a golden early chance, somehow firing over from three yards as he stretched for Aaron Holloway's deft touch.
But the home side broke through on 25 minutes when Paul Green crossed from the right and visiting defender Nathan Pond could only slice the ball past his own keeper at the near post.
It was almost 2-0 after 40 minutes as Ollie Banks hit a dipping free-kick which was well saved by Alex Cairns.
David Ball went close for Fleetwood, but Erwin deservedly scored 10 minutes after the break with a fine 12-yard volley from Holloway's assist.
Bobby Grant miscued a rare chance for Fleetwood before Oldham sub Michael Ngoo tested Cairns from the edge of the box as Fleetwood succumbed to a second loss in three matches.
Report supplied by the Press Association.
Fleetwood's League One automatic promotion bid looks all but over after they stumbled to defeat at Oldham.