There was nothing to split Pompey and Oxford over 210 minutes of what was often attritional football.
But that should probably not have come as a surprise considering the sides finished the suspended campaign level on 60 points.
In the end it took the lottery of a penalty shoot-out to send United to Wembley and leave Kenny Jackett’s men heartbroken.
Marcus Harness’ precise strike had given the visitors an advantage at the Kassam Stadium, only for an Ellis Harrison own-goal – awarded by technology – to level.
Cameron McGeehan was the unfortunate man to fail from the spot and Jackett must now prepare Pompey for another season in League One.
The Blues boss said: “They were two tight games where we’ve taken the lead twice, but again we’re disappointed with the goal we’ve conceded.
“It’s a mix-up at the back when it didn’t seem like there was any Oxford player around. We went on take control, but couldn’t capitalise on that.
"They were two tight games where we’ve taken the lead twice, but again we’re disappointed with the goal we’ve conceded."
“There were quite a few decisions by the referee – particularly in the first leg – that didn’t really work for us. You need those moments to go for you in these big games.
“We put lots of balls into the box and most of the time our opponents came up with the answers to that.
“There weren’t many chances across the 90 minutes and extra-time and there wasn’t a dominant side.
“We’ve been in the play-offs for two seasons running and just not quite been able to get over that hurdle.
“It’s incredibly frustrating, but our congratulations must go to Oxford and we wish them well at Wembley.”
Jackett has plenty of sympathy for loan midfielder McGeehan, who saw his penalty saved by Simon Eastwood.
He said: “Cameron’s inconsolable because it’s one penalty out of 10 when the others were all successful, but football is about how you get over that.
“He’s been a good player for us and has performed well over these two games. He’s given everything he’s got for the cause.”