For the second time in the space of a few days, Pompey scored three goals at Fratton Park.
But while they conceded the same number against Fleetwood at the weekend, Oxford only managed to find the net twice at a damp Fratton Park.
And Danny Cowley was delighted with the performance of his side to secure a valuable victory after falling behind to Marcus Browne’s early opener.
Sean Raggett head home an equaliser and George Hirst struck just before the break, while Hayden Carter added a long-range stunner in the second half.
Luke McNally reduced United’s deficit late on, but the hosts saw out the final few moments and Cowley revelled in the result.
The Blues boss said: “I’m pleased for everyone – the players, staff and, in particular, the supporters, who I thought were incredible.
“I was hoping for nights like that when I became Pompey manager and hopefully there are many more to come.
“We had a difficult start, but again we showed the character and resilience that was needed after falling behind.
“There was a tactical switch midway through the first half to get two up front because it was very wet and a challenging night for defenders.
"I’m pleased for everyone – the players, staff and, in particular, the supporters, who I thought were incredible."
“We wanted to put stress on their back three and managed to score a great header from Sean from a Joe Morrell cross.
“It was a lovely finish from George for the second and I thought he was immense – he was unplayable.
“As for the third – centre-halves aren’t meant to do that! It was a hell of a strike from Hayden and we should have had a fourth to make it easier for ourselves.
“But we defended well against a good Oxford team and they probably only had one decent chance, which they took, before we saw things out in a disciplined way.
“We had felt some injustice at the Kassam Stadium and if you stay honest, then football will reward you.
“It was the same for the supporters because they came to both games in their droves and if I’ve been involved in a game with a better atmosphere, I don’t remember it.”
Pompey were without Reeco Hackett, who is set for a lengthy spell on the sidelines after collecting an injury against Fleetwood.
Cowley said: “We’ve had some bad news. He took a bang to his knee on Saturday and it looks like he’s ruptured his lateral ligament.
“We’ll speak to the specialist and they will make a call on it. They tend to need surgery, but time will tell.”