Nicky Ajose scored twice as managerless promotion-chasing Bury left Oxford's play-off hopes in tatters.
The Shakers led within 60 seconds when Ajose lobbed Ryan Clarke from outside the area after Harry Worley failed to clear the ball.
Ajose set up Ryan Lowe for the second and he fired a shot that seemed to go through Clarke.
And 19-year-old Manchester United loanee Ajose sealed the win from close range.
The Shakers were led by caretaker boss Richie Barker, who was assisted by 26-year-old midfielder Steven Schumacher, following Alan Knill's departure to Scunthorpe on Thursday.
Bury caretaker manager Richie Barker told BBC Radio Manchester:
"It wouldn't matter who was in charge today, the players responded in a correct and professional way.
"I tried my best to enjoy it as it might be my one and only time I ever do it.
"It was exactly what everybody needed and hopefully it will kick-start another little run."