Pompey were in the lead and looking pretty comfortable as the Fratton Park contest passed the hour mark.
A firm header from Sean Raggett had broken the deadlock in the latest League One clash with Oxford.
But a triple substitution for the visitors breathed new life into the side and Alex Gorrin’s penalty ensured the spoils were shared.
And Kenny Jackett was disappointed that his troops could not take more of their opportunities when they were on top.
The Blues boss said: “We got ourselves into a good position and I did feel that we deserved the lead in the first half.
“There were other chances for us, with Jack Whatmough hitting the bar and John Marquis putting over a decent opportunity.
“It was a key period straight after the break – as it always is – and we were on top, without being able to put Oxford away.
"We got ourselves into a good position and I did feel that we deserved the lead in the first half."
“On the sidelines it did feel as if we needed that second goal and ultimately that proved to be the case.
“The changes that Oxford made definitely gave them energy and they got back into the game off the back of that.
“I was toying with making more myself, but then you’re taking off guys who are performing very well and are potential match-winners.
“We lacked a bit of energy in the middle as the game went on, but Tom Naylor and Andy Cannon have been big players for us.
“The contest went from end to end in the final few minutes and a chance for Ronan Curtis was possibly the most clear-cut one of the match.”
Pompey argued that Whatmough won the ball when Tim Robinson pointed to the spot for the defender’s challenge on Dan Agyei.
Jackett said: “One or two decisions didn’t really go our way, like the penalty or when Ellis Harrison was through and got fouled, but ultimately you have to accept the decision of the referee.”