Pompey Academy suffered their first Youth Alliance League defeat of the season, despite twice taking the lead against Oxford.
They played some flowing football in the first half at Furze Lane and deservedly took the lead through the lively Harrison Brook.
Adam Smith hit back to level before the break, only for Harvey Rew (pictured) to restore the home team’s advantage.
Connor Ferguson headed United back on level terms, but a foul on Brook then saw the Blues awarded a penalty.
Alfie Stanley’s effort was well saved by Max Evans, however, and Smith netted a late winner for Oxford.
Professional development phase coach Liam Daish said: “We played very well in spells and passed the ball around nicely to create a lot of openings.
“The game became more stretched in the second half and we had to deal with Oxford’s physicality, but we’re happy with how the lads are developing.
"It’s our first loss in the league this season and that shows how we’re progressing as a team."
“There might have been a shift in momentum after the penalty save, although you would always want Alfie stepping up in those situations and the keeper did well.
“Obviously the result didn’t go for us and there were little mistakes and lapses of concentration. Overall, though, we can’t grumble.
“It’s our first loss in the league this season and that shows how we’re progressing as a team. The players were disappointed afterwards, although I know they’ll bounce back.
“When you watch our goals lately, they’ve all come through really good play. Harrison has been excellent all season, while Harvey’s one came after a great overlap down the left. It’s fluid stuff.
“We’re leaving ourselves a bit open at times, but it would be easy to just get two banks of four set up and then hope for set-plays.
“I think that anyone watching our matches will appreciate what we’re doing. There are plenty of good prospects and the process is ongoing.
Pompey: Pitman; Kavanagh, Robb, Rew, Kelly; Bridgman, Storey (Teggart 60); Brook, Bell, Lee (Anderson 83); Stanley
Sub not used: Steward