Neither side had much to play for with United outside the play-off places and Stanley safe from relegation - and the first half felt like that.
In an opening period of few chances, nine-goal Lee Molyneux fired wide while Michael Liddle was denied by Oxford goalkeeper Max Crocombe.
Oxford's best efforts fell to winger Alfie Potter, who volleyed wide, while Sean Rigg's fierce shot was kept out by Stanley's Paul Rachubka on the stroke of half-time.
It was Leeds loanee Rachubka's last save of the season as highly-rated teenager Andy Dawber came on at the break.
However, James Constable fired low past him into the net for his 14th goal of the season from 12 yards out after 64 minutes having latched on to Potter's pass.
And three minutes later Constable was judged to have been fouled in the area by Peter Murphy, leaving Deane Smalley to send the ball into the bottom corner of Dawber's net.
Winger Molyneux saw a straight red after 89 minutes for a late challenge on Potter on the wing and, in the final minute, Oxford broke through and unmarked Sean Rigg tapped the ball home from eight yards out.