Myles Peart-Harris's goal helped Oxford earn a potentially crucial win over Watford
Oxford United moved within a point of Championship safety and extended their unbeaten run at the Kassam Stadium to five matches with victory over out-of-form Watford.
January signing Myles Peart-Harris slotted in from close range in the 19th minute, and Mark Harris sealed the victory two minutes into stoppage time to keep Oxford a point behind Portsmouth and move them three points above second-bottom Leicester City.
Portsmouth, in the last safe position, have a game in hand on both Oxford and Leicester.
In a match frequently lacking creative spark, Matt Bloomfield's Oxford forged the better chances against opponents who toiled for long periods despite making three attacking changes on the hour mark.
Watford were seventh in the table after winning at Bristol City on 27 February but are now 12th and nine points outside the play-off places with four games remaining after failing to win for a sixth time in seven games.
The U's had been on course to move out of the bottom three until Portsmouth scored a 97th-minute winner at Middlesbrough, making this result all the more important with four games remaining for the victors.
Signed on a short-term deal from Brentford, Peart-Harris' first home goal for the club was the first attempt on target of the game, with the forward pouncing after Watford could not conclusively clear Cameron Brannagan's delivery.
The Hornets had almost two-thirds of possession and six of the nine attempts between the teams during the first half but failed to trouble home goalkeeper Jamie Cumming before the break, returning to the dressing room at half-time to boos from some fans in the away end after struggling to find their tempo.
Luca Kjerrumgaard and Nampalys Mendy had efforts blocked as Ed Still's Watford looked dangerous on the break, although Oxford threatened when Stan Mills' cross gave Jamie Donley a presentable opportunity, only for the opening to disappear when the on-loan Tottenham Hotspur forward hesitated rather than shot.
Cumming made an excellent save shortly after the restart, reacting smartly to tip a long-range shot over the crossbar from Nestory Irankunda, who scored Watford's equaliser as part of an impressive display in their 1-1 draw at home to Charlton Athletic on Monday, and looked their likeliest candidate to create a telling move in this contest.
After Mills sent a free header wide from Brannagan's delivery, Egil Selvik pushed Brodie Spencer's shot from distance behind as Oxford seized momentum, Will Lankshear testing Selvik with another header.
Still responded by sending on Kwadwo Baah, Kevin Keben and Mamadou Doumbia with the hope of quickening Watford's work on the ball against opponents pushing to score a second.
Doumbia headed and shot wide from Irankunda passes, but Oxford were perhaps unfortunate not to earn a penalty kick for Edo Kayembe's challenge on Mills.
However, they secured victory two minutes into stoppage time when Harris entered the box and fired in after Watford lost possession in their own defensive third.
What's next for both teams?
Oxford will seek a first win in three away games at Derby County (12:30 BST) on 18 April, while Watford will seek to improve upon a run of two home draws when Sheffield United visit Vicarage Road (15:00) on the same day.
Reaction - 'It was imperative we won'
Oxford United head coach Matt Bloomfield told BBC Radio Oxford:
"I'm really after the performance and I'm really proud of the players for the manner in which we did it.
"When you look at the results, it was even more imperative that we won. As I said inside the dressing room, we can't worry about what happens elsewhere.
"We have to be positive, keep moving forward, and keep going after those performances, because it was a joy to watch us - especially in the second half - and we want four more of them."
Watford head coach Ed Still told BBC Three Counties Radio:
[On disgruntled fans] "I can understand it completely. It's the worst – our fans have travelled so well and have given us such support this season, and to send them home that way is horrible.
"They've seen a team that hasn't matched the opposition in terms of aggression and physical impact. That makes us all angry.
"It's not for being caught by surprise; we know what Oxford are, how windy it is, how the pitch is and how difficult it is, but you've got to deal with it. End of story.
"It is far too much of a familiar story from what's gone on in previous seasons, as well. To be repeating that trend now is terrible."
Still: 'I can understand it (the fans booing)'
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