Oldham continued their revival by charging to a 3-0 win at Boundary Park but the problems are mounting up for MK Dons.
Stuart Murdoch's side were never at the races as they suffered a fourth defeat from five league games.
Murdoch saw his team surrender to a quick Oldham double before half time and there was no way back after they conceded a third goal after 61 minutes.
The Latics had beaten Coca-Cola Championship outfit Stoke City in the Carling Cup during midweek to set up a plum tie at home to Tottenham and the buoyant mood was lifted further thanks to strikes from Jermaine Johnson, Chris Killen and Mark Arber.
Johnson could have scored after 28 minutes as he reached a pass from Mark Bonner but dragged his shot off target as the keeper rushed out to meet him.
But the Jamaican international made up for it 11 minutes later when he cut inside and hit an unstoppable shot past David Martin.
The second goal was made by captain Dean Holden who crossed from the right for Killen to head his first goal of the season from six yards out.
Johnson almost added a third after half time when he forced Martin into a reflex save but Oldham didn't have to wait much longer to kill of the game.
Arber provided the finish with his out-stretched left foot from eight yards after a free-kick from David Eyres.
The day was finished of for MK Dons when Malvin Kamara and Julien Hornuss had shots cleared of the line in injury time.