With results elsewhere going their way - Burton and Gillingham only drawing, and Walsall losing - Latics actually saw the gap to third spot extend to five points.
But they’ll know it could and should have been seven, only for Marcus Maddison’s 80th-minute equaliser which gave Posh a share of the spoils.
Will Grigg had looked like being the match-winner for Latics - who were far from their best - when he netted with trademark coolness with 18 minutes to go.
Posh had started the brighter, before Latics gained control of the game - without really working goalkeeper Stuart Moore enough.
The Latics boss will also be sweating on the fitness of goalkeeper Jussi Jaaskelainen, centre-back Jason Pearce and forward Conor McAleny, who were all substituted before the start of the second half.
And Chris McCann’s late booking was his 10th of the season, which triggers an automatic two-match suspension for the Irishman.