Oldham striker Chris Porter bagged a brace as League One leaders Nottingham Forest lost their 100% home record.
Oldham dominated proceedings and opened the scoring just before the half-hour mark when Porter tapped in after Paul Smith parried a Gary McDonald effort.
Porter went close again before he doubled the lead, nodding in after Smith misjudged Paul Edwards' cross.
Forest were lacklustre and Grant Holt shot into the side-netting as they were soundly beaten at the City Ground.
Nottm Forest: Smith, Gary Holt (Thompson 68), Morgan, Breckin, Bennett, Southall, Clingan, Harris (Dobie 75), Perch, Lester (Agogo 45), Grant Holt.
Subs Not Used: Padula, Pedersen.
Booked: Lester, Grant Holt.
Oldham: Pogliacomi, Charlton, Haining, Stam, Eardley, Wellens, Porter, McDonald (Rocastle 82), Edwards (Taylor 87), Liddell, Warne (Grabban 81).
Subs Not Used: Knight, Trotman.
Goals: Porter 29, 45.
Ref: D Drysdale (Lincolnshire).