Barnet joint-boss Mark Robson pointed to a change of personnel for Barnet's failure to apply their league form to the FA Cup.
The visitors were well worth victory as they registered 21 attempts on goal and hit the woodwork twice in the first half, but it took second-half goals from James Constable and Sean Rigg to make the game safe.
"I thought we were very poor. Oxford are a very good side and we never threatened them and we never hurt them," Robson said.
"We had a number of injuries before the game, we had to make five changes which is frustrating. You want to keep the side together that has been playing well recently but changes were forced upon us, and you have to say those who came in did not step up to it.
"We tried to change it after the break, we tried to press them higher up the pitch but no matter what we did it did not work. We didn't pass the ball well and we never got behind their defence and never created.
"Oxford are a strong side and a good club but we didn't do enough to test them and didn't do enough to win the game."