The pre-season favourites for promotion are sliding towards the League One drop zone after picking up just one win in 10 games and Forest fans turned on Megson during an abysmal second half for the visitors.
Oldham picked them off easily with goals from Richie Wellens, Richard Butcher and Luke Beckett, who all scored in the space of 29 minutes.
The vital three points kept Ronnie Moore's Oldham in the wide open hunt for a play-off spot.
The Latics also gained revenge for their 3-0 defeat at the City Ground last month when Forest had looked a completely different team.
Oldham gave a debut to teenager Chris Taylor on the right wing and the youngster gave an outstanding performance to be named man of the match.
He was involved in the first chance of the night, which saw Rune Pedersen deny Chris Porter.
Pedersen replaced Paul Gerard in goal after 14 minutes when Gerrard suffered a knee injury, but he was much the busier keeper and soon had to save from Wellens.
There were few clear openings in the first half but Oldham broke through on 53 minutes when a Butcher shot was deflected to Wellens, who stroked it into the far corner with his right foot from 10 yards.
Oldham's second came 20 minutes later as Butcher combined with Mark Hughes and burst into the box before slotting his shot into the bottom right hand corner.
Forest posed little threat up front and got virtually no change out of Oldham's three-man defence, despite the worthy efforts of Nathan Tyson.
The visitors' misery was complete after 82 minutes when substitute Chris Hall caused chaos in their defence and the ball broke for Beckett to side foot home his eighth goal of the season from eight yards.