An uneventful match boiled over in the 84th minute when a linesman ruled that Danny Reet's header, from a Richard Barker flick on, had crossed the line before Andrea Guatelli's save.
The keeper, on loan from Portsmouth, was booked for protesting that he had stopped the ball from crossing the line - and immediately Oxford fans raced on to the pitch, fighting with stewards and delaying the game for six minutes.
The goal stretched Mansfield's unbeaten run to six games, ended Oxford's four-match unbeaten run and denied them a deserved point after they had been thwarted almost single-handedly by veteran keeper Kevin Pressman.
In the first half Guatelli almost gifted the home side an 11th minute goal when he spilled a Jake Buxton cross, but John Dempster - a late replacement after Leo Roget was injured in the warm-up - cleared off the line.
The keeper redeemed himself with a full length save to keep out Gus Uhlenbeek's drive after a surging run from the experienced midfielder.
At the other end Pressman saved bravely at the feet of Steve Basham and two minutes later the striker headed over a Lee Mansell cross with the keeper beaten.
Three outstanding saves from Pressman then denied Oxford at the start of the second half. In the 56th minute he blocked Tcham N'Toya-Zoa inside the six-yard area and then made two fine stops in the 62nd minute.
Pressman then dived full length to tip away Chris Hargreaves' pile-driver and, from the resulting corner, his reflex stop somehow kept out a point-blank header from Chris Willmott.
The Stags thought they had snatched the lead against the run of play in the 70th minute when substitute Adam Birchall's tap in after a flowing four-man move was harshly ruled out for offside. But then came the late drama.