Oldham were knocked off top spot in League One as a controversial penalty gave resurgent Millwall victory.
Marvin Elliott [left] celebrates with match-winner Darren Byfield
After a goalless first half, the hosts took the lead on 55 minutes when Paul Warne headed in Neal Eardley's cross.
However, the lead lasted just six minutes as Marvin Williams slotted in after good work by Darren Byfield.
And the Lions snatched a seventh win in nine games when Byfield scored from the spot after Stefan Stam was adjudged to have handled Poul Houbertz's shot.
Oldham: Pogliacomi, Eardley, Haining (Pearson 86), Stam, Edwards (Lomax 77), Liddell (Hall 77), Rocastle, McDonald, Taylor, Porter, Warne.
Subs Not Used: Ryan Smith, Dabo.
Booked: Rocastle, Haining.
Goals: Warne 55.
Millwall: Pidgeley, Senda, Phillips, Shaw, Craig, Dunne (Hackett 21), Elliott, Brammer, Brighton (Williams 61), Byfield, Harris (Hubertz 60).
Subs Not Used: Day, Fuseini.
Booked: Dunne, Byfield, Brammer, Hubertz.
Goals: Williams 61, Byfield 82 pen.
Ref: S Bratt (West Midlands).