Posh's defeat saw them drop out of the play-off places and continued an abysmal run of away form as they have taken just one point from their last eight away trips.
The club gave a blanket "no comment" at the end of the game, with Ferguson refusing to send out any of his coaching staff.
Oldham produced an amazing second-half comeback to stun Posh at Boundary Park, with the winner coming from Genseric Kusunga in the fourth minute of stoppage time.
The visitors made it 3-0 at half-time as Lee Tomlin stroked in from 14 yards and Oldham - with only one victory in nine previous games - looked dead and buried.
But Gary Harkins launched the fightback from close range and David Worrall added Oldham's second with a fine strike before Posh made it 4-2 in the 63rd minute when Nicky Ajose punished a defensive blunder.
Johnson's side levelled the scores through James Wesolowski and an 89th-minute penalty by Harkins after Tomlin handled on the line. The Posh striker was shown a straight red card.
The clinching goal came after more chaotic defending by Peterborough, with Kusunga blasting in from two yards.
- Sports & Recreation
- Britt Assombalonga