United take a one-goal lead into the Play-Off semi-final second leg thanks to skipper Elliott Moore's powerful second-half header.
United named an unchanged starting line up from the side that won at Exeter last Saturday. Sam Long, who missed out at St James Park through illness, was on the bench.
The U's showed their attacking intent from the off, Josh Murphy winning a corner in the second minute and Ruben Rodrigues firing in a shot from 15 yards straight at Jed Steer a minute later.
For the first 10 minutes Oxford were largely in control without creating any clear-cut chances. It wasn’t until the 18th minute that Peterborough created their first attempt, with Ephron Mason-Clark’s shot from a narrow angle comfortably fielded by Jamie Cumming, who had been largely a spectator before that point.
As has been so often the case in recent months, Murphy was frequently the source of United’s most threatening moves, and on 34 he wove into the Peterborough area and let fly, his swerving shot just clearing Steer’s crossbar.
Another dangerous Murphy cross was headed just past his own post by Josh Knight as the U’s continued to turn the screw. A second corner a minute later saw Elliott Moore get his head to the delivery, but again it flew just over.
The first half belonged to Oxford United, but would they regret not making their dominance count?
Well, no. The second period was just six minutes old when Murphy sent in a deep corner. Headed back into the mix by Supporters' Player of the Year Cameron Brannagan, it was the skipper who rose highest to power home his header in front of the vociferous home fans.
Despite going a goal up, United continued to play to their game plan, pressing the Peterborough defence and keeping strong at the back. Moore, having scored his goal, made sure that he got his head to anything the Posh threw into the box, ably assisted by Ciaron Brown and the rest of the back line.
Towards the end, Oxford were defending their lead, happy to take an advantage to London Road on Wednesday. Ninety minutes from Wembley? Dare to dream.
Attendance: 11,125 including 1,478 away
Tonight's Fifty/50 draw was worth £1,459, won by ticket number: 138961
Report: Martin Brodetsky; Photographs: JasonPix, Steve Daniels, Steve Edmunds, Darrell Fisher