The Latics have taken the bold step of handing Johnson a new deal until 2018 d espite being in trouble near the foot of the table.
But they 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.
Johnson said: "It was an unbelievable game and such an important win for us.
"At half-time I wondered where to go, but some serious home truths were said in the dressing room and fair play to the lads for going out there and doing it all themselves.
"At half-time they were very embarrassed and I'd already made two substitutions at that stage because we couldn't get going.
"We were giving the ball away in silly areas when men were committed forward. When we lost it, Peterborough were crisp and slick and we couldn't deal with it.
"After that, what an unbelievable turnaround - it's probably one of the best games of football I've ever seen.
"We went more direct, we got more numbers up there and we just looked physically stronger.
"Gary Harkins was superb and for us and to score five goals in one half is brilliant. I hope it will be the catalyst.
"We've always known we had the quality in our team to score goals, but we haven't been showing it.
"Hopefully a performance like this one will unleash the beast."
Peterborough's defeat saw them drop out of the play-off places and continued an abysmal run of away form.
Posh have now taken just one point from their last eight away trips and gave a blanket "no comment" at the end of the game, with manager Darren Ferguson refusing to speak to the media or send out any of his coaching staff.
- Sports & Recreation
- Gary Harkins
- Lee Tomlin