Two first half goals from Hector Kyprianou and Ephron Mason-Clark set Posh on the way to a battling victory in the capital.
The margin of victory could and perhaps should have been more, but Peterborough United got the three points and on this day, that is all that mattered. It was Hector Kyprianou, against his former club for the second time this season, and Ephron Mason-Clark who found the net in the first half and despite a fine goal for the hosts, Posh clung on for all three points.
It was a bit of a shaky start to the game with the O's dominating the opening exchanges, however on eight minutes, Posh found the breakthrough as a fumble from goalkeeper Sol Brynn allowed KYPRIANOU to cushion home from close-range on the volley.
Mason-Clark fired wide at the near post after good run on 19 minutes before Ricky-Jade Jones failed to make contact to a David Ajiboye centre.
Harrison Burrows saw a long-range free-kick gathered by the keeper on 23 minutes before Posh doubled the lead as MASON-CLARK stooped to force the ball over the line from close-range after an intelligent header back across goal by Knight.
Mason-Clark was kept out by Brynn on 37 minutes after a strong run before the same man fired a shot over the bar via a deflection.
Jones failed to divert a Burrows cross towards goal on 53 minutes and Kyprianou guided a shot wide from the edge of the box before Orient pulled a goal back as GALBRAITH guided a superb volley into the top corner.
Jones had an immediate chance to restore the two-goal cushion after robbing Beckles, but his shot was saved by Brynn's legs. Malik Mothersille was denied by Brynn moments later after a quickly-taken free-kick before Jed Steer produced a superb stop to keep out a deflected Brown shot.
A minute from time, substitute Agyei had a golden chance to level, but he headed over from a corner kick when all alone.
Leyton Orient: Brynn, James (sub Hunt 46min), Sweeney, Brown, Cooper, Moncur (sub Sanders 56min), Forde (sub Sotiriou 46min), Beckles, Adu-Adjei (sub Pigott 46min) O’Neill (sub Agyei 82min), Galbraith. Unused subs: Howes, Edwards.
Posh: Steer, Burrows, Edwards, Knight, Katongo, Kyprianou, Collins, Poku, Mason-Clark, Jones (sub Mothersille 69min), Ajiboye (sub Randall 69min). Unused subs: Bilokapic, Crichlow, De Havilland, Olakigbe, Clarke-Harris.
Half-time: Leyton Orient 0-2 Peterborough United
Full time: Leyton Orient 1-2 Peterborough United
Referee: Sunny Singh Gill
Attendance: 8,658 (1,200 Posh fans)