James Constable had given Oxford a first-half lead but goals from Rene Howe, Ryan Burge and Chris Zebroski gave County their first win since Boxing Day.
It moves Justin Edinburgh's side to within six points of the League Two play-off spots with three games in hand over seventh-placed Southend.
Constable had given the visitors the advantage in the 14th minute when he smashed a left-foot shot past Elliot Parish after a superb ball from Dave Kitson.
The home side had to wait until the second minute of first-half injury-time to draw level when Howe deflected in Lee Minshull's long-range shot for a goal on his home debut.
Burge then gave County the lead with a stunning free-kick after 70 minutes before a defensive mix-up allowed Zebroski to grab the home side's third.
Substitute Alfie Potter scored for Oxford in the 87th minute but Edinburgh's side held on for a vital win.