Erwin gave Latics the lead on 11 minutes after he volleyed home at the far post but with 20 minutes of the game remaining Junior Morais headed the equaliser as Latics had to settle for the point.
Manager John Sheridan made one change to the side which beat Oxford United on Tuesday night.
Aiden O’Neill replaced the injured Rob Hunt as Latics lined up in a familiar 4-4-2 formation.
It would be Peterborough United who had the first chance of the game as Michael Smith was denied by Connor Ripley.
The Posh full back managed to skip inside on the right and fired low but Ripley saved with his legs.
Latics came back into the game and it would be Erwin who had the first chance for Latics.
Aaron Amadi-Holloway laid the ball into the path of the striker but his effort from 25 yards went wide of Mark Tyler’s goal.
It wasn’t long after, however, when Latics took the lead.
Ollie Banks’ ball into the box found Erwin who twisted his body and volleyed past the goalkeeper to give Latics the lead and score his third goal in two games.
That goal kicked Peterborough United into gear and they started to take a grip of the game.
Andrew Hughes headed wide whilst a minute later, Banks got back to block Craig Mackail-Smith’s effort from inside the penalty box.
Midway through the half, Ripley was forced to make another superb stop, this time from Mackail-Smth.
The striker was allowed space in the penalty area but Ripley got down to his left to turn the ball around the post.
From the resulting set piece, Tom Nichols volleyed over the crossbar from six yards.
The Posh enjoyed a lot of the possession but their final ball into the box was lacking.
When they did deliver a good ball into the box through Smith on the right, no Posh man attacked the ball as Athletic came away with the ball.
Before the half time break, Paul Taylor fired over the crossbar from inside the penalty area, and minutes later, Mackail-Smith stretched to shoot but could only fire wide.
Latics started the second half brightly and almost doubled their lead three minutes into the half.
A free kick was awarded after O’Neill was fouled on the edge of the box, Chris Taylor stepped up but his free kick curled wide of Tyler’s goal.
Posh defender Jack Baldwin was the first player to enter the referees notebook on 55 minutes after a late challenge on Erwin.
Latics had to be alert just after the our mark as McLaughlin cleared he ball off the line.
Mackail-Smith headed towards goal, it hit an Oldham Athletic man but McLaughlin was at the back post to clear.
Tope Obadeyi replaced the injured Aaron Holloway just after the hour mark for Latics as Posh brought on Morais.
The first piece of action for the Posh forward was to enter the referee’s notebook after he fouled Banks late.
However, the striker did manage to equalise for Peterborough United on 73 minutes.
A cross into the box from Chris Forrester found Morais in the centre who headed powerfully past Ripley to equalise for Posh.
Athletic had a great chance to regain the lead only seconds after the restart.
O’Neill played Obadeyi through on goal but the striker snatched at his chance and fired wide.
Posh had two opportunities to take the lead soon after as Brad Inman curled wide of Ripley’s far post whilst Mackail-Smith set up Morais but he struck wide from 18 yards.
It would be Latics who were the most threatening in the final 10 minutes of the game, however they were dealt a blow as Anthony Gerrard limped off on 82 minutes.
Latics had their best chance on 85 minutes as Obadeyi was played through on goal but Smith just managed to get back and prevent the striker from getting a clean shot away.
O’Neill’s effort was blocked minutes later whilst Ngoo headed over at the back post before six minutes of additional time was announced.
Banks had the final chance of the game in the final minute of additional time but struck wide from the edge of the box as Latics had to settle for a point.
Peterborough United: Tyler, Smith, Hughes (Binnom-Williams 64’), Bostwick, Baldwin, Forrester, Taylor (Maddison 46’), Mackail-Smith, Inman, Nichols (Morias 59’), Grant.
Subs not used: McGee, Lopes, Anderson, Penfold.
Oldham Athletic: Ripley, McLaughlin, Clarke, Gerrard (Reckord 82’), Wilson, Green, Banks, O’Neill, Taylor, Holloway (Obadeyi 65’), Erwin (Ngoo 82’).
Subs not used: Kettings, Law, Fane, Flynn.
Goals: Peterborough United (Morais 73’) 1-1 Oldham Athletic (Erwin 11’)
Attendance: 4,222 (332 visiting supporters)