Peter Leven's late penalty saw Oxford come from behind to beat League Cup semi-finalists Bradford in the Bantams' first game since beating Aston Villa.
Nahki Wells capitalised on Wayne Brown's error to tap in, only for Sean Rigg to fire home a powerful equaliser.
Matt Duke continued his excellent form to deny Leven and Alfie Potter as Oxford grew into a combative game.
Duke tipped Rigg's header onto the bar, but was finally beaten by Leven's spot-kick after Potter was tripped.
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