Gateshead led 2-0 deep into the second half until the high-flying fourth tier side snatched two goals in the last 14 minutes, and an equaliser in the 89th minute, to earn a 2-2 draw and a replay in the north-east.
James Marwood 's stunning 30-yard strike gave The Heed a 14th-minute lead.
Marwood, the son of ex-Arsenal midfielder Brian Marwood, drove a brilliant shot over U's keeper Ryan Clarke and into the top left-hand corner to fire Gary Mills's team in front.
Jamie Chandler added a second for Gateshead in the 51st minute when he latched onto a misplaced pass by Scott Davies to slot home.
Deane Smalley pulled a goal back from close range for Oxford in the 76th minute.
And Danny Rose headed in an equaliser from two yards out in the last minute, after Adam Bartlett had blocked Dave Kitson's volley, to save the Football League side's blushes.
Gateshead were left kicking themselves. They could easily have added to their early lead with Oxford skipper Jake Wright making a brilliant last-ditch tackle to deny Colin Larkin after goalkeeper Clarke and Tom Newey collided.