Cameron Brannagan’s 88th-minute free-kick brought Oxford a 1-0 victory over Peterborough for the second game running – and a place in the second round of the Carabao Cup.
Brannagan, who also hit the winner in Saturday’s League One game, shot low into the bottom left corner from just outside the box after Niall Mason brought down Jamie Hanson.
Both teams made a host of changes following their league meeting on Saturday, with U’s boss Karl Robinson bringing in seven fresh faces, while Darren Ferguson made five changes.
Ivan Toney kept his place in the Peterborough attack, and he stood out in the opening quarter of an hour.
Toney had one goal disallowed for offside and struck a left-footed volley over from 10 yards after a neat chest trap.
The striker then forced a parried save from Simon Eastwood with a first-time shot from 15 yards.
Oxford’s young forward Malachi Napa was stretchered off on 21 minutes following a crunching block tackle, and the home side struggled to mount attacks until Sam Long tried a shot just before the break.
Ben Woodburn was denied by keeper Christy Pym saving with his legs at the near post.
Posh nearly broke the deadlock when a left-wing cross from Serhat Tasdemir sliced off the foot of Rob Dickie and hit the far post.
Eastwood also made two smart saves late in the second half to help Oxford progress.