James Constable's 100th goal for Oxford United has given his side a deserved half-time lead at 10-man Mansfield Town.
The 29-year-old finished neatly from close range deep into eight minutes of stoppage time following Ryan Williams's pinpoint cross.
Sean Rigg had earlier broken the deadlock on 13 minutes, before Lee Stevenson's third goal in four games pulled the Stags level in the fifth minute of added time.
Shortly after Rigg's opener, Mansfield midfielder Jamie McGuire was shown a straight red card.
Rigg's opener came against the run of play, as Mansfield had made a strong start at Field Mill.
Samuel Clucas, who scored four second-half goals as the Stags won 8-1 at St Albans in the FA Cup last week, went close to finding the net again early on, but his shot from long range deflected just wide.
However, once Oxford had taken the lead, Chris Wilder's men, who are unbeaten away from home in the league this season, began to dominate.
Rigg struck the crossbar with a mishit cross, before things got much worse for Mansfield on 23 minutes when McGuire was dismissed for a two-footed tackle on Danny Rose.
McGuire has now been sent off seven times during his career, while Mansfield have received four red cards in four league games.
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