Posh took the lead through former Orient man Aaron McLean on nine minutes when he turned home a cross from the left but the O's deservedly pulled level shortly before the break thanks to a well-taken Shaun Batt goal.
The visitors raced back into the lead immediately after the break through Oztumer's brilliant curling free-kick from 20 yards and Orient never managed to recover during a very scrappy second-half.
It was the first time these two sides have met since that unforgettable night in May when Dean Cox and Chris Dagnall’s goals were enough to secure a 2-1 victory for Orient at Brisbane Road, sending them through to the League 1 Play-Off final but there was to be no repeat of that uphoria for the O's today as Peterborough did what they had to do to reverse that result.
Both sides came into the match in similar form with Orient winning one of their last six in the league and Peterborough without a victory at all in the same number of games, so this was always going to be a close game between two sides that were almost impossible to separate last season too.
It was Fabio Liverani’s first game in charge of the O's since being appointed less than a week ago and the new boss made three changes to the side that was knocked out of the JPT by Gillingham in Mauro Milanese's final game as manager last Saturday.
With four strikers currently missing through injury or suspension, and Jay Simpson only passed fit enough to make the bench, Liverani went with the untested strike partnership of Batt and Dean Cox, with the latter asked to drop deep and link midfield with attack.
Having lost at home to Colchester United last week, Peterborough boss Darren Ferguson also made a trio of changes, sending out a side that featured four ex-Orient players: Ben Alnwick, Gabriel Zakuani, Harry Beautyman and McLean.
In the opening stages, Orient's makeshift striker Cox seemed to be settling in well to his new role and took just three minutes to threaten the Peterborough goal when McAnuff broke down the left flank before drilling a cross into the middle. Cox was there to turn the ball goalwards and beat the keeper only for Kgosi Ntlhe to get back on the line and heroically turn the ball behind for a corner.
However it was the visitors who took the lead against the run of play on nine minutes through one of the former O's men, McLean. The striker peeled off the back of Nathan Clarke and managed to stay onside well before latching onto Joe Newell's cross from the left, guiding it beyond Adam Legzdins.
Orient remained the better side after the goal, without completely dominating proceedings, but were finding it difficult to breach the visitors' defence despite some promising build-up play in the middle third.
And as the half progressed the game was descending into a scrappy affair with both sides giving away countless free-kicks in the midfield which led to nothing.
But just as it looked like the break would arrive before either team came anywhere near threatening the scoreline again, Orient pulled level on 39 minutes.
Cox found himself in a more familiar left-wing position and drove at the defence to send a low ball across the 18-yard box where Pritchard had come in from the right flank and laid the ball into Batt's who composed himself before sending a low left foot shot past Alnwick.
Whilst occasionally looking isolated for the O's up top, Batt had put in a very good shift for his side and deserved his goal, which was the first he's got since February.
The visitors should have re-taken the lead shortly before the break when a powerful cross from the left was dropped by Legzdins leaving Harry Beautyman a half-empty goal to aim at however he could not direct his effort on target, letting his former club off the hook.
Orient may have escaped that time, but Posh wasted no time in pushing for the lead again after the break and found themselves back in front within two minutes of the re-start.
McLean drew a foul out of Clarke right on the edge of the O's penalty area and Oztumer stepped up to bend a pinpoint free-kick into the top right-hand corner of the net with Legzdins unable to get anywhere near it even at full stretch. It was a tremendous strike however it looked to be a soft free-kick decision from referee Kevin Johnson in the first place.
Having worked tirelessly for an hour, Liverani decided to replace Batt with Simpson to inject some more energy into Orient's attack and the former Arsenal striker sent a looping header towards the top corner soon after coming on, but Alnwick moved his feet well to get across and deny Simpson.
Just as the opening period did, the second half turned into a largely ugly spectacle as the final whistle drew closer and in attempt to find some spark, first-year scholar Victor Adeboyejo was handed his debut by Liverani while Scott Cuthbert was thrown up-front in a late effort to conjure something from the game.
However, it was looking highly unlikely that the O's were going to get anything from it, with Peterborough doing an effective job in slowing the play down and getting bodies behind the ball.
But in the final action of the half, Orient came within inches of securing a dramatic equaliser through debutant Adeboyejo. Newell brought down Josh Wright - subsequently earning his second booking of the game - 40 yards from goal to give the home side one last throw of the dice.
Legzdins was sent forward by Liverani for the free-kick and the confusion it brought to the Posh defence almost paid off when Adeboyejo got on the end of Vincelot's knock-down to prod it goalwards but Alnwick was adjudged to have stopped the ball from crossing the line despite the appeals of O's fans and players alike.
Seconds later, Johnson brought an end to a fairly turgid game of football, but it was not one that the O's deserved to lose which will provide Liverani with hope going into next week's game away to Barnsley - particulalry with a number of key players set to return from injury and suspension.
ORIENT: Legzdins; Cuthbert, Lowry, Clarke (C), Sawyer; Pritchard, Wright, Vincelot, McAnuff (Adeboyejo, 84); Cox, Batt (Simpson, 63)
SUBS: Woods, Baudry, Omozusi, Lee, Kashket
GOALS: Batt (39)
BOOKINGS: Cuthbert (29), Sawyer (88)
P'BORO: Alnwick, Bostwick, Ntlhe, Burgess, Zakuani, Payne, Beautyman (Anderson, 86), Newell, Oztumer, McLean, Mackail-Smith
SUBS: Day, Smith, James, Vassell, Washington, Mendez-Laing
GOALS: McLean (9), Oztumer (47)
BOOKINGS: Oztumer (36), Payne (50), Newell (72,97)
SENT OFF: Newell (97)
REF: Kevin Johnson
ATT: 5,546 (916)