A debut goal from 17-year-old Robert Hall was enough for Oxford to win at Dagenham.
Hall, signed from West Ham on Monday, collected Paul McLaren's pass and fired home from just inside the area.
Alfie Potter mised two great chances to extend Oxford's advantage but the Daggers could have levelled through Mark Arber's volley.
The home side had another chance to equalise but Sam Williams' header was well saved by Ryan Clarke.
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U's manager Chris Wilder told BBC Oxford:
"It's possibly the hardest place that he (Robert Hall) could have made his debut. He's a local lad and they know a lot about him here.
"They made it difficult for him, they wanted to make contact with him but he grew into the game, it was a great goal. And in the second half, he was a real handful.
"He's given us that little bit of spark."