Pompey 1 Oxford 4
Pompey were handed a harsh introduction to life in League Two after conceding four and having a man sent off against Oxford.
It looked like it might be a perfect start when Patrick Agyemang headed the hosts ahead midway through the first half.
But Deane Smalley and Alfie Potter took advantage of some slack defending to ensure it was United who went ahead at the interval.
It was always going to be an uphill battle when Johnny Ertl was red carded following a challenge on Danny Rose.
And Oxford made their extra man count as both the impressive Potter and Smalley added to their tallies.
Guy Whittingham handed debuts to John Sullivan in goal, Sonny Bradley and Joe Devera in defence and Andy Barcham and Simon Ferry in midfield.
There were also a host of familiar faces on show, with former Blues players Dave Kitson and Ryan Williams in the visitors’ matchday squad, while ex-Blues and Us boss Jim Smith watched from the directors’ box.
The sides came out to a rapturous reception from the sell-out crowd, with fans in the Fratton End holding up cards to spell out the word ‘Ours’.
Pompey looked bright in the opening stages and Jed Wallace was first to go close with a shot from distance that flew narrowly over the bar.
Agyemang also saw an effort deflected behind before Bradley glanced a header wide from a corner.
The Blues then won a corner on 20 minutes following a fine passing move involving Barcham, David Connolly, Ferry and Wallace, but Agyemang could only head the resulting cross over.
It was Oxford who went close to breaking the deadlock moments later, however, with Potter just failing to connect with Andrew Whing’s volley at the far post.
That let-off sparked the perfect response from Pompey, who took the lead on 24 minutes.
Ertl started the move off by winning a tackle in the centre and feeding Barcham, whose cross from the left was headed home by AGYEMANG.
Sullivan was called into action for the first time soon after, getting quickly down to his left to grab hold of Whing’s fierce low drive.
The keeper was powerless to stop the visitors from equalising on 34 minutes, though. Potter beat Dan Butler down the right and sent in a low cross for Smalley to convert.
And the Blues soon found themselves behind, with Sean Rigg dispossessing Yassin Moutaouakil and squaring for Potter to slam home.
Half Time: Pompey 1 Oxford 2
There was a slow start to the second half as Pompey struggled to find a way through the Oxford back-line.
And their hopes of finding an equaliser were dealt a hammer blow on 58 minutes when they were reduced to 10 men.
Ertl was shown a straight red card after referee Gavn Ward adjudged the Blues skipper to have used his elbow in a challenge with Rose.
Whittingham immediately reshuffled his pack, bringing on Ricky Holmes for Butler and switching to three at the back.
But Oxford immediately exploited their numerical advantage, with Potter finding plenty of space to run towards goal and drill a shot past Sullivan.
Pompey came close to reducing the deficit on 66 minutes when Wallace’s corner led to a scramble in the visitors’ box, but Ryan Clarke was able grasp hold of the ball.
The Fratton faithful did have something to cheer soon after, even if it was only for a late challenge on pantomime villain Kitson that saw Bradley receive a booking.
But it was the Oxford fans celebrating on 71 minutes when Devera failed to cut out Potter’s cross, allowing Smalley to stroke home a fourth.
Before that Whittingham had made a double switch, with Romain Padovani and Tom Craddock replacing Wallace and Agyemang.
It might have got even worse for the Blues on 75 minutes, but Johnny Mullins’ header came back off the post.
Craddock did have an opportunity to open his account against his former side after Clarke could only parry Barcham’s shot.
But the striker was unable to readjust his feet quickly enough as the ball raced towards him and could only fire wide.
And that was as close as Pompey came to ending a disappointing afternoon on a high note.
Pompey (4-4-2): Sullivan, Moutaouakil, Devera, Bradley, Butler (Holmes 62); Wallace (Padovani 69), Ertl (c), Ferry, Barcham; Connolly, Agyemang (Craddock 69)
Goals: Agyemang 24
Sent off: Ertl
Subs not used: Smith, East, N’Gala, Bird
Oxford (4-4-2): Clarke; Hunt, Mullins, Wright (c), Newey; Potter, Rose, Whing, Rigg (Davies 89); Kitson (Hall 83), Smalley (Constable 74)
Goals: Smalley 34, 71, Potter 38, 64
Subs not used: Crocombe, Raynes, Williams, Ruffels, Davies
Referee: Gavin Ward
Attendance: 18,181 (2,500 away fans)