The League Two rivals must now face each other a further three times during December as they clash in a league match on Tuesday and also meet in the area quarter-finals of the LDV Vans Trophy.
This was the archetypal 'game of two halves' with the first 45 minutes dominated by Cheltenham and the second going the way of Oxford.
In fact, so one-sided was the first half that the home side could have been three or four goals up at the break.
Striker Steve Guinan put them ahead when he produced a neat piece of skill inside the box on 14 minutes. Latching onto a Brian Wilson cross from the left, he twisted and turned away from his marker before clipping a low shot beyond goalkeeper Billy Turley and inside the far post.
Turley saved brilliantly from Kayode Odejayi on three occasions, touching one lob shot over the bar and turning aside two low drives, and both Odejayi and JJ Melligan smacked the crossbar while Guinan had one cleared off the line by Matt Robinson.
The second half was a completely different story with Oxford drawing level after 52 minutes when Eric Sabin met a Lee Bradbury cross with a glancing header that bounced just inside the far post.
After that, Sabin hit the crossbar with a drive from 18 yards and the Cheltenham goal had another incredible let-off with 66 minutes played.
Steve Basham played in midfielder Adam Griffin down the left and his shot struck the bar when it appeared easier for him to score. The rebound fell to Chris Hargreaves who also struck the woodwork and the teams were forced to settle for a place in the third round draw but a replay to decide who goes through.