Mia Adaway scored her first senior goal on an otherwise disappointing afternoon for Pompey Women.
The substitute’s late strike proved no more than a consolation at the PMC Stadium, as the hosts fell to a 4-1 defeat.
Jay Sadler made two changes from the side that beat Gillingham in the National League Cup the previous week.
Charmaine True and Roxy Lee-Stewart both returned to the starting line-up, with Tamsin de Bunsen and Laura Ingram making way.
But the Blues fell behind after just seven minutes, as Kayleigh Hines looped a shot into the net from outside the box.
They recovered well from that setback and started to see plenty of the ball, without really troubling Aimee Watson in the United goal.
A 20-yard drive from Becki Bath was easily dealt with by the keeper and that was as close as Pompey came to an equaliser.
Oxford were more clinical, however, and doubled their advantage when Evie Gane tapped home following a scramble in the box.
Half Time: Pompey 0 Oxford 2
It was the visitors looking more threatening once the action restarted and a volley from Cheryl Williams grazed the crossbar.
Mel Burns then prevented further damage with a fine double stop to deny Georgia Timms on the hour mark.
Pompey were trying to find a route back into the contest and Jade Widdows fired wide after her initial effort had been saved by Watson.
A threaded pass from de Bunsen then played True through on goal, but a last-ditch tackle prevented the striker from getting a shot away.
And Oxford soon increased their lead, as a long-range effort from Emily Allen was deflected into the net.
A fierce strike from ADAWAY late on restored some Pompey pride, but it was United who had the final say.
They scored their fourth of the contest deep into stoppage-time, with Williams lifting the ball over Burns.
Pompey: Burns; Tonks, Southgate (c), Rowe, Currie; Lee-Stewart, Bradley; Widdows (Adaway 86), Bridge (de Bunsen 68), Bath; True
Sub not used: Younger