Ivan Toney struck his 19th goal of the season as Peterborough snatched a 1-0 victory at Oxford's Kassam Stadium.
Marcus Maddison caught out the home defence with a 76th-minute free-kick to Toney on the right, and the unmarked striker fired an unstoppable shot across goalkeeper Simon Eastwood and into the net.
It brought Darren Ferguson the first win of his third spell as Peterborough manager and dealt a big blow to Oxford, who slipped back into League One's bottom four.
Posh had been quick out of the blocks, with Toney forcing Eastwood to tip over his header from their first corner, then Jason Naismith smashing a right-footed drive against a post in the fifth minute.
The visitors lost on-loan centre-half Josh Knight, who was making his debut, to injury after only 21 minutes.
Cameron Brannagan went close for Oxford with a right-footed shot from Gavin Whyte's cut-back and brought out a good save from Conor O'Malley, diving to his left.