Robert Hall scored the only goal as Oxford made it three away wins in a row by beating Hereford.
Oxford's James Constable and Hereford's Sam Winnall both failed with headers before the game's only goal.
Hall made it two goals in four since his loan switch from West Ham, heading home from close range after Liam Davis took on two men and crossed the ball.
Alfie Potter could have made the win more comfortable but missed a sitter in the second half.
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U's boss Chris Wilder told BBC Oxford:
"I'm a bit disappointed that we didn't give ourselves a clearer winning advantage and made the last 25 minutes a little easier.
"But we handled it when it got chucked at us late on and we always looked a threat going forward so I'm delighted with everybody.
"I think we always try to play the right way, playing through teams."