Posh, beaten 5-1 at Bury in their previous away game, were defeated 2-0 by a side that only had 13 League One goals all season prior to the match.
"We were poor from minute one. The Bury result was a 5-1 defeat, but we never folded like (against Oldham)," he said.
"That's probably the worst I've seen us under my management. I'm disgusted with the performance to be honest."
Second-half goals from Paul Green and Ryan McLaughlin, their first for the Latics, gave the second-from-bottom hosts victory over a Posh side currently just four points outside the play-off places.
McCann, who became manager in May, continued to BBC Radio Cambridgeshire: "Without me being too critical, because I'll take responsibility, I'm the manager, on the whole the character this season has been good. It wasn't against Oldham.
"We lacked energy, tempo, anything about us. We were launching hopeless balls into their box into centre-halves like Anthony Gerrard and Peter Clarke who've been around this league and just head things away.
"I toyed with making changes to freshen it up, but I gave them another shot because they won the game. I'm possibly regretting that decision now."
Peterborough United manager Grant McCann has said "everything" went wrong in the defeat by Oldham on Tuesday.