Sirike Dembélé was the hero for United this afternoon as his first goal for the club in the 82nd minute earned the U's their first win since January against a Watford side with play-off aspirations. Oxford were helped on their way with the sending off of James Abankway for a foul on Przemysław Płacheta having already been booked.
Gary Rowett made five changes to the side that lost at Hull City on Wednesday, with Ben Nelson starting in central defence following the injury to Elliott Moore and a fit-again Cameron Brannagan coming into midfield in place of Alex Matos.
Elsewhere, the three attacking midfielders were all changed as Płacheta came in for Stan Mills, Ole Romeny was preferred to Rúben Rodrigues, and Dembélé took Matt Phillips' place. Tyler Goodrham overcame the illness that prevented him being involved at the MKM Stadium to take his place on the bench.
For the first time this season Oxford found themselves attacking towards the East Stand in the first half and they were quick out of the blocks, pressing well and looking to get on the front foot wherever possible.
Oxford's first sight of goal came in the 14th minute, when Płacheta was fed on the right wing and he cut inside, dragging his shot just wide from the edge of the area.
There was a scare for United when Tom Ince played Moussa Sissoko through the defence, but Cumming spread himself well and the ball rebounded off the Watford player for a goalkick.
Just three minutes later, Oxford had their own opportunity to open the scoring as Nelson rose to meet a Brannagan corner but just couldn't keep the ball down. Tom Dele-Bashiru had the next effort, his shot from distance straight at Cumming.
The second half was less than five minutes old when Oxford carved out an excellent opportunity. Płacheta's burst down the right took him beyond the last defender and his cross was met by Dembélé, who steered the ball just the wrong side of the post.
Watford were reduced to 10 men with 10 minutes to go following a second yellow card for Abankwah. United took immediate advantage as Dembélé raced onto a ball in the penalty area, kept his cool, and steered the ball beyond Egil Selvik for what could turn out to be a priceless goal.
There was plenty of tension in the final minutes, especially with seven added on for stoppages, but United held firm and will go into the international break with three massive points under their belts.
UNITED: Cumming, Brown, Vaulks, Płacheta (Phillips 89), Brannagan, Harris, Romeny (Goodrham 68), Nelson (Kioso 68), Dembélé (Matos 89), ter Avest, Helik
Unused subs: Ingram, Long, El Mizouni, Rodrigues, Mills
Attendance: 11,535 with 1,519 from Watford
Fifty/50 draw: Ticket number 43904 wins £1,506.50
Report: Martin Brodetsky; Photography: JasonPix, Steve Daniels, Steve Edmunds, Darrell Fisher