Oxford were given a scare by Braintree before finally overcoming the National League side in an FA Cup first-round replay 3-1 at the Kassam Stadium.
The away side took the lead through a Kenny Davis penalty after Joe Skarz had fouled Michael Cheek. But League Two United hit back before half-time through Liam Sercombe.
And two second-half goals from Pat Hoban - the second a penalty following a foul on Callum O'Dowda - sent Oxford into round two to face Forest Green.
Watch all the goals from the FA Cup first round here.