Hall, 17, who only joined on loan from West Ham on Monday, collected Paul McLaren's pass and beat Chris Lewington with a low shot from just inside the area after 41 minutes.
Dagenham failed to create much in the first half and it was the visitors who went close to opening the scoring when indecision in the Daggers defence allowed Simon Heslop to volley goalwards but Lewington saved with his feet.
After the break the hosts pushed forward and Mark Arber saw his volley sail wide before substitute Brian Woodall failed to connect from two yards out.
Alfie Potter missed two glorious chances to make the points safe for Oxford and the home side could have earned a late equaliser when Sam Williams headed goalwards from Abu Ogogo's right-wing cross, but Us goalkeeper Ryan Clarke was equal to the effort.
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