Harvey White was on target as Pompey impressively returned to winning ways by beating in-form Oxford.
They looked in control for long periods at the Kassam Stadium and the only question was whether they could make a breakthrough.
But it arrived early in the second half, with White converting a Callum Johnson delivery for his first Blues goal.
And Craig MacGillivray was relatively untroubled until right at the end, with the keeper making three excellent stops to ensure the points would be heading back to the south coast.
There were three changes from the side that were beaten by Blackpool on home soil at the weekend.
White came in for his first start since arriving on loan from Tottenham in January, while there were returns for Lee Brown and Ellis Harrison.
Ryan Williams, Charlie Daniels and John Marquis were the men to make way, with the trio dropping to the bench.
Ronan Curtis, meanwhile, moved to a central attacking role alongside Harrison, as White took up position on the left.
The visitors started brightly and a long MacGillivray clearance caused problems before the ball eventually ricocheted into Harrison’s path.
But United keeper Jack Stevens was alert to the danger, tearing quickly off his line to make a vital stop.
There were not an abundance of opportunities at either end, though, with half-chances the order of the first period.
Sean Raggett and Johnson made important blocks in quick succession, while a fierce James Henry strike was beaten away by MacGillivray.
But it was Pompey looking more likely to break the deadlock as the interval approached, courtesy of some neat passing.
A stretching Curtis could not quite get enough on Johnson’s dangerous delivery to properly test Stevens.
Andy Cannon then saw an effort deflected wide following some impressive work from the lively White to win back possession.
Half Time: Oxford 0 Pompey 0
The second half was not even three minutes old when the Blues broke the deadlock, with WHITE opening his account for the club.
He raced between a couple of defenders to guide home a fine header after Johnson had delivered a first time cross from the right.
Oxford tried to hit back immediately, although Elliot Lee’s free-kick flew safely into the arms of MacGillivray.
Pompey were not content to sit on their lead, however, and Harrison had a shot blocked before Cannon’s volley was dealt with by Stevens.
The visitors made their first change just past the hour mark, as Williams replaces Marcus Harness wide on the right.
And the newcomer was involved in a neat passage of passing play soon after coming on, ending with him not quite able to sneak a shot inside Stevens’ post.
United went close to levelling on 74 minutes, with MacGillivray off his line to deny Olamide Shodipo, but unable to hold onto the ball.
Sam Winnall looked to capitalise, only for the quick-thinking Raggett to get there first and hack the ball to safety.
A second switch immediately followed, as Marquis came on for Curtis to offer the visitors some fresh legs up front.
He soon saw an angled effort kept out by Stevens, just moments after White had benefitted from good work by Williams and had a shot blocked.
Jordy Hiwula then replaced Harrison for the closing stages, as the Blues appeared to be seeing the contest out with relative ease.
But a late flurry of Oxford chances almost brought an equaliser, with MacGillivray coming to the rescue on each occasion.
He beat away a thunderous Anthony Forde shot, tore off his line to deny Shodipo and then tipped over an Elliott More header right at the death.
Oxford (4-1-4-1): Stevens; Long, Moore (c), Atkinson (Forde 84), Ruffels; Brannagan; Lee (Barker 65), Henry (Taylor 75), Sykes (Gorrin 65), Shodipo; Winnall
Subs not used: Eastwood, Grayson, Kelly
Pompey (4-4-2): MacGillivray; Johnson, Raggett, Nicolaisen, Brown; Harness (Williams 63), Naylor (c), Cannon, White; Harrison (Hiwula 86), Curtis (Marquis 75)
Goals: White 48
Booked: Curtis, Harness
Subs not used: Ward, Whatmough, Daniels, Byers
Referee: Graham Salisbury