Playing three up front Posh carried the game to the Londoners, who looked to have weathered the storm with John Mackie and teenager Gabriel Zakuani impressive in the middle. But Orient fell behind in first half added time as Calum Willock put injury agony behind him to celebrate his first start of the season.
Former Fulham striker Willock met Paul Carden's chip into the area to send a header looping over goalkeeper Glyn Garner for the first goal conceded by Orient in more than six hours of action.
Ordered out early by manager Martin Ling for the second half, this was the signal for the former League Two leaders to unleash a tremendous barrage of pressure on the home goal.
Posh needed to dig deep to stay in the game and had to thank goalkeeper Mark Tyler for a tremendous fingertip save that denied Orient's top scorer Gary Alexander.
Livewire Michael Simpson managed to beat 10-season veteran Tyler with a fierce shot only for a post to come to Posh's rescue, the ball coming off the woodwork and skidding along the line before being cleared to safety.
Next it was Joe Keith's turn to be denied, this time with Tyler tipping away a dangerous effort from a set-piece.
Orient finally clawed their way level on 74 minutes with long serving Matt Lockwood lifting in a free-kick that defender Miller met and steered past Tyler into the bottom right-hand corner of the net.
With their unbeaten away record saved, Orient went all out for a winner and only another superb Tyler save prevented Simpson from grabbing the decisive goal.
The final 10 minutes also brought chances at the other end with teenage substitute Jamie Day showing a lot of potential for Posh with fellow sub Trevor Benjamin and Carden going close.