Australian Adrian Segecic scored on his debut to give Portsmouth a 1-0 victory in a fiercely contested Championship encounter at Oxford.
The summer signing struck in the 38th minute with a sharp follow-up finish after Jamie Cumming saved from Colby Bishop, who had broken clear from Cameron Brannagan's poor backpass.
Pompey are now unbeaten in their last eight league games against the U's.
John Mousinho's men went into the match without their ex-Oxford winger Josh Murphy but were boosted by striker Bishop's presence, along with the that they had lost only one of their previous 11 opening day league fixtures.
Mousinho handed three players their debuts - John Swift, Florian Bianchini and Segecic.
Skipper Elliott Moore was making his 200th league appearance for the U's, who had summer signings Brodie Spencer and Will Lankshear making their first appearances for the club.
However, fellow new signing Brian De Keersmaecker was unable to join them because of a knock in training.
Pompey started brightly and a brilliant run down the right by Callum Lang, then a deep cross, gave a difficult chance beyond the far post for Bianchini which he could not direct at goal.
Both sides were guilty of sloppy passes in a tight encounter where any space was at a premium.
The first effort on goal in the game came from the visitors in the 21st minute when Cumming comfortably saved Bianchini's low 15-yard shot.
U's boss Gary Rowett was shown a yellow card for apparent dissent from his technical area.
The home side produced their best attack of the first half when Brannagan was set free on the left and he drilled in a left-footed drive that Nicolas Schmid turned around his post.
Pompey had spent the bulk of the first 45 minutes in Oxford's half so it came as no surprise when Segecic, signed from FC Sydney, gave them the lead.
United started the second half better and Stanley Mills tested Schmid at his near post with a fierce shot from a tight angle.
And as the game opened up, Segecic cut in from the right at the other end and fired over.
Lankshear had a headed effort ruled out for offside as the U's pressed more, attacking towards their own fans.
Jack Currie's shot from the edge of the box was tipped over by Schmid in the closing minutes, and Louie Sibley blazed over from the corner.
But Portsmouth held out for their clean sheet to pick up three points at the Kassam Stadium for the second season running.
The managers
Oxford's Gary Rowett:
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Portsmouth's John Mousinho:
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