Myles Peart-Harris scored their opener in the 19th minute when the Hornets were unable to clear a Cameron Brannagan free-kick.

And substitute Mark Harris sealed a first win in five games on a fast break two minutes into stoppage time with a low shot past goalkeeper Egil Selvik.

Matt Bloomfield’s team looked to be climbing out of the relegation places, but Portsmouth’s late, late winner at Middlesbrough leaves them in 22nd place with four games to go.

Watford, who have a poor away record this campaign, were on a winless streak of three going into the game but they had beaten the U’s 2-1 at Vicarage Road at the start of October.

Jeremy Petris showed power and skill to get around the back of Oxford left-back Brodie Spencer in the 17th minute of a cagey opening, but his pullback was well defended.

Brannagan’s fine free-kick delivery from the right created United’s opening goal. Michal Helik and a defender both seemed to get a touch in the air and the ball dropped invitingly for Brentford loanee Peart-Harris to slot home.

The Hornets responded with a sustained spell of pressure, and Luca Kjerrumgaard planted a header straight into keeper Jamie Cumming’s clutches.

Stanley Mills proved an important outlet on the right flank for the U’s and when he slipped a pass inside for Jamie Donley, the Northern Ireland international lost his footing at the vital moment.

A powerful run from Edo Kayembe set up an opportunity for the visitors just before the break but full-back Sam Long was alert to the danger to head a dangerous cross just past his near post for a corner.

Watford knew they had the players capable of getting them back on level terms, and at the start of the second half Cumming had to tip over a 22-yard effort from Australia international Nestory Irankunda.

Yet Oxford often looked threatening from their set pieces, with Brannagan and Donley delivering pinpoint free kicks and Peart-Harris causing havoc with his long throws.

From one throw, Brannagan saw a shot deflect just wide.

Selvik had to save from Brodie Spencer, Will Lankshear, Donley and Yunus Konak as the U’s kept up their intensity and pressed hard for a second goal.

On a Watford counter-attack, Mamadou Doumbia glanced a header wide from Irankunda’s cross.

The tension among the home fans in the closing stages when Watford pressed for an equaliser was palpable – but Wales international Harris settled it emphatically.