Oxford gave their hopes of beating the drop in the Championship a boost with a 2-0 win over mid-table Watford.
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.
Oxford's Matt Bloomfield:
"It was a massive result for us but we're not getting carried away. We're still below the (relegation) line and still have a lot of work still to do. But we've given ourselves a real chance and I think we would have taken that a few games ago.
"The most important thing was getting three points today but I thought the manner in which we played was extremely satisfying, especially second half.
"In the first half it was hard for us against a strong wind. We got our noses in front and then ended up sitting in a little because we couldn't get out.
"But to play with the energy and intent that we did in the second half was great, it felt like we were on the front foot.
"It would have been nice to have got the second goal earlier to make it a bit more comfortable, because at 1-0 with the calibre of individual players that Watford have, it can turn at any moment.
"We wanted to get another clean sheet because we built that platform when we arrived and haven't had one for a little while. We had a few big players, big characters step forward today."
Watford's Ed Still:
"We haven't been good enough. We haven't matched Oxford in terms of aggression, in terms of adapting to this type of game at this time of the season.
"Everything becomes more intense, more power-based and we haven't been able to deal with it well enough.
"To lose the game on a set piece, second ball - a very similar goal to the match against Charlton only a few days ago - and for it to be killed off by a mistake from us at the end there is extremely frustrating and makes you angry.
"I think everybody felt we weren't able to match them, and to create sufficient moments in the game where we could dictate the play."
