It is all to play for in the second leg at the Matchroom Stadium after Peterborough United and Leyton Orient drew 1-1 at London Road.
Inside the opening two minutes, the visitors were inches away from breaking the deadlock as a Kevin Lisbie cross just eluded strike partner David Mooney at the back post.
The deadlock was broken on 16 minutes and it was a superbly made goal, Swanson's initial effort parried by goalkeeper Jones, Little provided a superb cross-shot across the six-yard box and ASSOMBALONGA stooped to head home from close range.
Alcock was wrongly cautioned on 18 minutes for a foul, it appeared to be Ntlhe that did the damage as Odubajo raced down the right.
Ajose fired wide from a well-worked free-kick on 32 minutes as both sides battled to contend with the heavy winds. Vincelot fired wide from 30-yards as Posh struggled to clear.
Bostwick was well positioned to clear after a Cox header as Olejnik missed his punch following a long throw. In stoppage time, Olejnik produced a fine stop to deny Cox as the O's caught Posh on the counter attack.
Assombalonga was denied by Jones after excellent work from Bostwick on 49 minutes. Posh made their first change moments later with Conor Washington replacing Ajose.
Skipper Tommy Rowe fired inches past the post on 53 minutes after a good run and pick out from Swanson.
On 68 minutes, Orient made a double substitution with Shaun Batt and Lloyd James introduced from the bench.
Lloyd Isgrove replaced Swanson on 70 minutes as Darren Ferguson made his switch. On 72 minutes, the visitors found an equaliser as Moses ODUBAJO tapped home from close-range after Olejnik parried Mooney's shot into his path.
Olejnik produced a fine save diving to his right to tip a low Cox effort around the post on 75 minutes. Olejnik denied Cox on 83 minutes as the O's continued to dominate.
Odubajo fired inches past the far post with a left-foot half volley on 86 minutes with Olejnik beaten.
Final Score: Posh 1-1 Leyton Orient
Posh: Olejnik, Alcock, Ntlhe, Baldwin, Bostwick, Swanson (sub Isgrove 70min), Payne (sub McCann 81min), Assombalonga, Rowe, Ajose (sub Washington 51min), Little. Unused subs: Day, McQuoid, Knight-Percival, Newell.
Leyton Orient: Jones, Omuzusi, Vincelot, Cuthbert, Baudry, Cox, Lisbie (sub Batt 69min), Mooney, Odubjajo, Clarke, Lundstram (sub James 69min). Unused subs: Larkins, Sawyer, Bartley, Dagnall, Ness.
Attendance: 9,519 (1,710 Orient fans)