Chester were left stunned as Oxford pulled off a win in a hotly-contested game that saw 11 yellow cards.
The visitors took the lead when a cross by Lee Bradbury took a deflection and fell kindly to Mark E'Beyer, who hooked the ball in for his first goal.
Chester levelled when Kevin Ellison side-footed home.
Matt Robinson gave Oxford the lead after his shot took a deflection before Steve Basham fired home after good work by substitute Craig Davies.
Chester: MacKenzie, Edmondson, Hope, Bolland, McIntyre, Drummond, Davies, Harris, Booth, Rapley, Ellison.
Subs Not Used: Brown, Vaughan, Hessey, Lathom, Whalley.
Booked: Hope, Bolland, McIntyre, Drummond, Davies, Rapley.
Goals: Ellison 35.
Oxford Utd: Tardif, Mackay, Roget, Ashton, Robinson, Basham,
Wanless, E'Beyer, Quinn, Bradbury, Mooney (Davies 75).
Subs Not Used: Hackett, Brooks, Clarke, Molyneaux.
Booked: E'Beyer, Wanless, Quinn, Roget, Basham.
Goals: E'Beyer 16, Robinson 67, Basham 84.
Att: 2,791
Ref: N Miller (Co Durham).