Second-half goals from Karen Carney and Emily Westwood maintained Blues’ 100 per cent record in the competition after three fixtures.
However, David Parker’s side were made to work for this win by their FA WSL2 opponents who put in a resolute defensive display.
Parker had made five changes to the starting line-up that beat Manchester City Women in the FA WSL on Sunday.
The hosts, making five changes to the starting XI that beat Manchester City Women in the league on Sunday, dominated the first-half but it was credit to Oxford that the scoreline remained goalless at the break.
Carney, Kirsty Linnett and Hannah Keryakoplis all chipped away at the yellow-shirted backline but Demi Lambourne, in the Oxford goal, comfortably dealt with everything that came her way.
In an attempt to make the breakthrough in the second-half, Parker introduced Remi Allen and the vastly-experienced Jo Potter at the break in place of Keryakoplis and Coral-Jade Haines.
However, it wasn’t until the 63rd minute until that breakthrough came at the Autotech Stadium as skipper Carney stepped up to send a well-struck 20 yards free-kick into the top left corner.
That looked to have opened up the floodgates; Carney dinked an effort on to the roof of the net before Westwood swept a low drive from the right just wide of the far post.
However, it took another 12 minutes for the second to arrive. Carney’s corner from the right found the head of Kerys Harrop and the ball fell kindly for Westwood to hammer home from yards out.
England international Carney had played her part and was withdrawn for Charlie Wellings for her senior debut.
Substitute Allen almost stretched the lead further in the 76th minute but, having raced through down the left, poked a low shot wide of the far post.
Blues starting XI: Baggaley, Sargeant, Harrop, Keryakoplis (Potter H/T), Linnett, Carney (C) (Wellings 75), Simpkins, Moore, Westwood, Torkildsen, Haines (Allen H/T).
Subs not used: Spencer, Weston, George.
Oxford United starting XI: Lambourne, Osborne-Ricketts (Bruce 83), Chivers, Stanley, Allison, Low (Nichol 64), King, Nutman, Connor-Iommi, Fellows (Franklin-Fraiture H/T), Lane.
Subs not used: Cox, Pinke.