Pompey had fought from behind to lead Oxford in the closing stages at the Kassam Stadium.
Goals from Colby Bishop and debutant Callum Lang, who had emerged from the bench, meant all the momentum was with the visitors.
But a sloppy conclusion to the contest allowed James Henry to grab a late leveller for United and ensure the spoils were shared.
There was a lot to please John Mousinho, but he was unhappy with how his side performed at the start and finish.
The Blues boss said: “You can break the game down into three stages – I thought we were poor in the first half and then really good for most of the second.
“We were on the front foot and deserved to be leading, only for six minutes of madness right at the end.
“Conceding that equaliser was so unlike us, but even after that, we didn’t do the right things and made poor decisions.
“All the pressure had been building and building on Oxford, we got ahead and then we just threw it away.
"We were on the front foot and deserved to be leading, only for six minutes of madness right at the end."
“Whenever we found our midfielders before the break we looked a threat, but we just kept coming back.
“The message at half-time was that we weren’t showing enough bravery on the ball and when we went backwards, Oxford pressed us.
“It was completely different afterwards. We were still patient, but when the opportunity arose to get into the box, we took it.
“That impact from Callum is why we brought him to the football club and he looked lively and threatening.
“But we didn’t lock the game out properly, we allowed their player to have a one-v-one with our centre-half and they’ve got a goal from a rebound.
“I don’t mind if sides are able to open us up, but the two goals we’ve conceded on this occasion are just awful.”
Pompey announced another addition to their squad shortly before kick-off, with defender Tom McIntyre signing from Reading.
Mousinho said: “It’s a real boost for us because we’ve wanted to have four centre-halves following the injury to Regan.
“We’ve got an experienced Championship player who brings a wealth of experience and quality with him.”