Oxford's Michael Raynes headed home a dramatic equaliser with the last move of the match to deny Accrington Stanley an FA Cup victory.
Stanley thought they had sealed their place in round three when Lee Molyneux curled home a sublime free kick in the final minute to put the hosts 3-2 up.
But the visitors levelled deep into stoppage time when Peter Leven crossed for Raynes to equalise.
It was a frantic end to a pulsating cup tie in which Stanley twice thought they had scored late winners.
After Jon-Paul Pittman and Craig Lindfield traded first half goals, James Beattie's 30-yard half volley looked to have won the game 10 minutes from time only for James Constable to equalise on 86 minutes.
But Stanley will still be in the draw for round three with the replay at the Kassam Stadium set to be played on Wednesday week.