Pompey’s four-match winning run came to an end, as Oxford scored a late National League winner at Court Place Farm.
A Katie James free-kick cancelled out Georgia Timms’ opener, only for Emily Allen to strike for United at the death.
Jay Sadler made two changes from the side that secured a 2-0 victory at Gillingham the previous week.
Alex Collighan – back from international duty – replaced sister Caitlin, while Jazz Younger made way for Ellie Kirby.
It was an even contest before the break, although a shot from Ellie Noble struck the post in the opening stages.
And the Blues had an opportunity to take the lead after Jade Widdows nutmegged a defender, only for her dangerous cross to be cleared.
Half Time: Oxford 0 Pompey 0
Instead it was the hosts who were handed an excellent chance to break the deadlock just four minutes after the restart.
Rebekah Tonks challenged Timms inside the box and after consulting with his assistant, the referee pointed to the spot.
It was Timms who stepped up to take the penalty, with the forward sending Hannah Haughton the wrong way.
But the Blues levelled with a set-piece of their own soon after, as an excellent 35-yard free-kick from JAMES hit the roof of the net.
Becki Bath was introduced from the bench on 65 minutes and had a low strike saved by United keeper Lucy Thomas.
Both sides were chasing a winner and Thomas also denied Jade Bradley, while Flo Fyfe had a couple of chances for the hosts.
And it was Fyfe who had a role in Oxford’s second goal, switching the play for Allen, who hit a first-time effort that nestled in the bottom corner.
That proved enough for United, who deployed some time-wasting tactics to see the game out and claim all three points.
Pompey: Haughton; Kirby (Younger 82), Southgate (c), Tonks, Currie; Rowe, Bradley, James; A.Collighan, Albuery, Widdows (Bath 65)
Subs not used: Perkins, May, C.Collighan
Difficult one to take, especially with the timing of the goal conceded. Can’t fault the girls work rate, we could only control so much. Shows how far we’ve come as a group though, and more importantly the next steps we must take to continue our growth! #Pompey #BlueTogether 🔷👊🏻— Jay Sadler (@Sadler004) April 14, 2019