Watford and Oxford last played each other in the league in 1998-99 - a season in which the Hornets were promoted from the second tier and the U’s were relegated.
Both sides had limited time to prepare for Friday's game - Oxford beat Hull City 1-0 at home on Tuesday and Watford lost by the same score at Swansea the same night.
Towards the end of a quiet first half, Oxford worked the first real decent goalscoring opportunity, Peter Kioso picking up the ball on the right and curling a cross to the back post where Dane Scarlett slid in but could not quite poke the ball home.
The hosts nearly took the lead just before the break, Festy Ebosele crossing the ball into the box to Imran Louza, who caught it sweetly on the half-volley but his effort was deflected behind for a corner.
Watford nearly got off to the perfect start after the interval as Ebosele was played clean through on goal but, as he was set to shoot, the retreating Kioso put a hand on his shoulder and the Hornets man lost his bearings, failing to get a shot away.
The home crowd were screaming for a spot-kick but referee Anthony Backhouse was not satisfied Kioso had interfered enough with the Hornets forward to warrant a foul.
The home side then took the lead in the 53rd minute, counter-attacking with pace too quick for the U's defence to deal with.
Kwadwo Baah sprinted down the right before cutting the ball across to the waiting Bayo whose first shot was well saved by Cumming, only for the Watford forward to fire home a successful second attempt despite the goalkeeper's best efforts.
Bayo looked set to finish the game off with less than 10 minutes remaining as the ball was crossed to him with space and time in the box but Cumming was quick to react and saved well again.
Oxford went close late on as Mark Harris sent a close-range overhead kick over the bar but remain 16th with 17 points.