Centre-half Andrew Davies powered home a header from Gary Jones' corner seven minutes into the second half. And it was from another Jones flag-kick which Nahki Wells flicked the ball in from close range after 68 minutes, with an assist from Davies.
The Bantams are already celebrating their best start to a home league campaign for more than half a century, but they had found points hard to come by on the road.
Yet even in a dull first half of few chances they looked the more capable side.
Will Atkinson caught Ryan Clarke way out of his goal and almost embarrassed the 'keeper with a 30-yard shot from near the left touchline, before top scorer James Hanson might have done better than plant an eight-yard header wide from Kyel Reid's cross.
Injury-hit Oxford offered very little going forward until Peter Leven's introduction just after the hour mark. James Constable then blasted a shot over on the turn, and Leven himself brought a fine low save from John McLaughlin after curling a 20-yard free-kick around the wall.