Steve Basham scored twice and Dean Whitehead hit a brilliant 25-yard free-kick as United had it wrapped up by half-time against Northampton.
Basham, back after missing two games with an ankle injury, struck after only 58 seconds when ex-Cobblers midfielder James Hunt chipped over the visitors' defence and he ran through to score.
Admittedly, he looked offside.
But there was no disputing Whitehead's stunning shot from a free-kick on 32 minutes which put United two up. It arrowed into the top right corner.
Oxford's midfield of Paul Wanless, Whitehead and Hunt dominated, and Basham got his second of the game and seventh of the season on 41 minutes.
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