A powerful second-half display from Pompey saw them progress in the Johnstone’s Paint Trophy tonight.
Goals from sub Patrick Agyemang and John Marquis did the damage as the Blues picked up a 2-1 win at the Kassam Stadium.
James Constable pulled a goal back late on but Guy Whittingham’s side held on to progress to the third round of the competition for the first time.
Pompey were looking to continue the momentum created by Saturday’s excellent win over Rochdale.
But it was turgid stuff in the first 45 minutes with the visitors failing to create a chance of note.
Bondz N’Gala was alert to clear Alfie Potter’s cross after three minutes.
Then Trevor Carson did superbly to deny Constable’s header with a reaction save in the 15th minute.
Pompey were then forced into a change as Shaun Cooper replaced the injured Marcos Painter at left-back.
N’Gala did excellently to deny Constable before Michael Raynes headed over from a corner when well placed in the 28th minute.
Johnny Ertl then got caught on the ball which allowed Oxford to work the ball to Josh Ruffels, who fired over from 20 yards.
The feeling was it had to be better from Pompey after the restart.
And it was for the 1,132 away fans as they cheered their team’s first shot in the 54th minute – a scuffed effort from Romain Padovani at Ryan Clarke.
Carson again saved Pompey after 62 minutes when he denied Constable with another reaction stop.
But it was better from the visitors as they signalled further intent with Ertl flashing a 25-yarder just wide
Marquis was then inches away from opening the scoring with a dipping volley.
And the Pompey pressure paid off in the 65th minute when sub Agyemang made the breakthrough, with a lovely curled finish from 20 yards with the outside of his boot.
Pompey could have had a second on 73 minutes when Holmes found space in the box but Clarke kept out his finish.
Marquis then should have killed the game off when he went clean through with 10 minutes left but Clarke won their duel.
He got his second Pompey goal moments later, though, with a clinical header from a lovely Yassin Moutaouakil cross.
Pompey gave their fans a nervy last few minutes when Constable pulled one back late on.
But Whittingham’s side held on to progress to the third round.