High-flying Huddersfield were brought down to earth as Oldham grabbed a deserved 1-0 win.
Oumare Tounkara's goal proved decisive and Ian Bennett had to make a string of saves to keep Huddersfield in the game.
Impressive Oldham created the first chances as Dean Furman and Chris Taylor were both foiled by Bennett's double block, before they made the breakthrough after 24 minutes.
Tounkara ran on to Furman's through ball and kept his composure before slotting home from 10 yards.
Oldham took almost total control and Kieran Lee, Tounkara and Taylor all missed good chances.
Huddersfield created little apart from a header wide by substitute Alan Lee and Bennett made another fine save from Cedric Evina before Oldham's Jean Yves M'Voto saw his header cleared off the line.