Oldham eased past sorry Forest thanks to a rousing second-half display.
Wellens scored Oldham's opener
A dull opening period saw neither side create a decent opprtunity but the Latics went ahead after the break when Chris Taylor teed up Richie Wellens.
Richard Butcher made it 2-0, jinking past two defenders and firing low into the net from the edge of the box.
Luke Beckett sealed the victory when he took advantage of a defensive lapse to ensure Gary Megson's Forest have now won only once in 10 games
Oldham: Lee Grant, Swailes, Branston, Stam, Hughes, Wellens, Butcher, Edwards, Chris Taylor (Haining 90), Beckett, Porter (Chris Hall 75).
Subs Not Used: Day, Tierney, Anthony Grant.
Booked: Swailes, Branston.
Goals: Wellens 53, Butcher 73, Beckett 82.
Nottm Forest: Gerrard (Pedersen 13), Morgan, Breckin, Curtis, Bennett, Southall, Clingan (Perch 76), Thompson, Lester (Commons 45), Tyson, Grant Holt.
Subs Not Used: Fernandez, Bastians.
Ref: J P Robinson (E Yorkshire).