Sean Raggett’s goal was not enough to earn victory for Pompey, as they were held to a draw by Oxford at Fratton Park.
It was probably not a surprising outcome given that all four meetings between the sides last term finished all square.
The hosts looked comfortable after Raggett’s firm header had broken the deadlock, but their opponents were invigorated by some switches in the second half.
And Alex Gorrin levelled for United from the penalty spot, while both teams then had chances to win the contest.
Kenny Jackett made two changes from the side that eased to a comfortable victory over Crewe at the weekend.
They both came in the full-back berths, where Cam Pring and Haji Mnoga made way for Lee Brown and Callum Johnson.
Former Oxford winger Ryan Williams was fit enough to appear on the bench following a hip injury, while ex-Blues stopper Simon Eastwood started for the visitors.
It was a fairly nondescript opening to the contest, with nothing to trouble either keeper in the opening stages.
The hosts were happy to let their opponents see plenty of the ball and then look to hit them on the break.
And Pompey took the lead with their first chance of the game on 19 minutes when RAGGETT decided to stay up after a Jack Whatmough free-kick had been partially dealt with.
Ronan Curtis swung the ball back in the area, where the defender got in front of three players to firmly head home.
They probably should have doubled the lead soon after, but John Marquis uncharacteristically fired high and wide from Ellis Harrison’s centre.
It was the Blues looking more likely to add to the scoring and when Whatmough met a Marcus Harness cross, his header cannoned back off the bar, with Harrison unable to force in the loose ball.
The striker then thought he had been brought down by Elliott Moore as he tried to race through on goal.
Both players glanced across to the officials, with the defender likely to have been shown a red card if referee Tim Robinson had not waved play on.
Half Time: Pompey 1 Oxford 0
The visitors had to deal with a couple of dangerous left-sided deliveries from Curtis and Brown following the restart.
Pompey were working hard all over the pitch, with Curtis epitomising that by tracking back and halting a dangerous foray forward from Sean Clare.
They were not doing much to trouble Eastwood between the sticks, but the hosts were looking comfortable.
Karl Robinson responded by making a triple substitution and one of the newcomers, Marcus McGuane, immediately drew a fine save from Craig MacGillivray.
The Blues soon hit back at the other end, as Harrison met Brown’s cross with a neat flick that Eastwood tipped over the bar.
But United had a chance to level on 69 minutes when Robinson deemed Whatmough’s challenge on Dan Agyei to be foul, despite the defender protesting that he won the ball.
And Gorrin made no mistake from the spot, coolly sending MacGillivray the wrong way to draw the visitors level.
Pompey looked to restore their lead and a fine delivery from Tom Naylor was met by Curtis, who screwed his shot wide.
Andy Cannon and Harness made way for Bryn Morris and Williams, as Jackett injected some fresh energy into his midfield for the closing stages.
But it was the Oxford substitutes causing more problems and MacGillivray had to palm away a low Mark Sykes effort, with the ball thankfully not falling for the lurking Matty Taylor.
The Blues also had opportunities to turn one point into three, however, and Williams had two efforts blocked before Raggett glanced a header wide.
Curtis also headed Johnson’s delivery straight at Eastwood, as neither side managed to find a winner during a frantic finish.
But there was a melee after the final whistle, with Taylor escaping punishment for grabbing Curtis in an uncomfortable area.
Instead it was the Pompey winger that received a yellow card, along with Oxford defender John Mousinho.
Pompey (4-4-2): MacGillivray; Johnson, Whatmough, Raggett, Brown; Harness (Williams 82), Cannon (Morris 80), Naylor (c), Curtis; Marquis, Harrison
Goals: Raggett 19
Booked: Cannon, Raggett, Curtis
Subs not used: Bass, Nicolaisen, Pring, Close, Hiwula
Oxford (4-1-4-1): Eastwood; Long, Moore, Atkinson, Ruffels; Gorrin; Clare (Mousinho 86), Henry (c) (Sykes 62), Forde (McGuane 62), Shodipo (Obita 62); Agyei (Taylor 82)
Goals: Gorrin 69 (pen)
Booked: Clare, Taylor, Mousinho
Subs not used: Stevens, Kelly
Referee: Tim Robinson