Bradford claimed their first away league win of the season with victory at struggling Oxford, who fell to a fifth successive defeat.
Andrew Davies headed a powerful opener from Gary Jones' corner and another Jones corner led to Nahki Wells flicking the ball in from close range.
Peter Leven went close in reply, forcing a fine low save from Jon McLaughlin with a 20-yard free-kick.
But Oxford were unable to prevent their worst run under manager Chris Wilder.
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Oxford United manager Chris Wilder told BBC Radio Oxford:
"We gave ourselves a platform at half-time, I didn't think it was a fantastic performance but it was one after recent results that we could look to kick on.
"Goals change games and we've conceded two from set pieces that we could have done better with. From then on in it was always going to be a struggle. We were lacking confidence and options in a lot of areas.
"It's a tough gig at the moment, I'm trying to get through it and we are trying to stick together and stay strong."