The U's currently have a slender advantage after claiming a 1-0 victory in Saturday's first leg at the Kassam Stadium.
Match preview
Peterborough suffered a 1-0 defeat in Saturday's first leg, leaving them at risk of suffering playoff semi-final disappointment for a second consecutive season.
Despite being edged out by the U's, Posh would have been relieved to keep the deficit to one after falling to a heavy 5-0 loss in last month's away league game against Oxford.
Posh will now return to the Weston Homes Stadium, where they have racked up 55 goals in 23 home games to help them finish the campaign as the league's highest scorers.
They have showcased their firepower in their last three home matches, thrashing Port Vale 3-0 and hammering Fleetwood Town 4-1 before playing out an entertaining 3-3 draw with Bolton Wanderers.
Peterborough also produced a free-scoring display in December's home game against Oxford, with Hector Kyprianou, Ricky-Jade Jones and Ryan De Havilland getting on the scoresheet in a resounding 3-0 victory.
Oxford will carry momentum into Wednesday's fixture after they beat Exeter City to creep into the playoffs on the final day of the season, before they backed up that result with a successful outing in Saturday's home leg against Peterborough.
Club captain Elliott Moore scored the decisive goal in the 53rd minute, heading home from Cameron Brannagan's corner to give Oxford the all-important lead ahead of the midweek trip to the Weston Homes Stadium.
Although they may have the upper hand, the U's still have significant work to do if they are to reach the playoff final for the first time since 2019-20.
The visitors can take confidence from the fact that they have put together a four-game unbeaten away run (W3, D1), conceding no more than one goal in any of those four matches.
Oxford's hopes of protecting their narrow lead could depend on whether they score the first goal, having avoided defeat in all 11 of their away league games where they opened the scoring (W9, D2).
Peterborough United League One form:
L
W
W
L
D
L
Oxford United League One form:
W
W
L
D
W
W
Team News
The hosts are unable to call upon the injured duo of Jacob Wakeling and Jeando Fuchs, but the latter could be available if Peterborough are able to secure their place in the playoff final.
Kwame Poku is pushing for a starting berth after recovering from injury to feature as a half-time substitute in Saturday's first leg.
Having scored three goals in Peterborough's last two home games, Malik Mothersille could keep his place up front despite being replaced just before the hour mark at the Kassam Stadium.
As for the visitors, Oxford manager Des Buckingham has reported no fresh injury concerns, ensuring that he will have most of his squad available for selection.
The 39-year-old is likely to opt for consistency and stick with the same lineup that started the narrow first-leg victory.
Welsh forward Mark Harris will continue to spearhead Oxford's attack after scoring three goals in the last two away games.
Peterborough United possible starting lineup:
Steer; Katongo, Knight, Edwards, Burrows; Kyprianou, Collins; Poku, Randall, Mason-Clark; Mothersille
Oxford United possible starting lineup:
Cumming; Stevens, Moore, Brown, Bennett; Dale, Rodrigues, Brannagan, Goodrham, Murphy; Harris
We say: Peterborough United 2-2 Oxford United
Peterborough's array of attacking talent will undoubtedly cause Oxford problems in Wednesday's second leg, but with the U's unbeaten in four away matches, we think that they will hold Posh to a draw to secure their place at Wembley.