A late goal by substitute Aaron Webster clinched Burton a 1-0 win against Oxford and a second round tie at home to League Two rivals Chesterfield.
Despite nursing a calf strain, Webster came off the bench to head the only goal of the game in the 85th minute.
Burton dominated the first half but persistent pressure was firmly resisted by a resolute Oxford defence.
Russ Penn, Shaun Harrad and Jacques Maghoma all failed to hit the target for the home team, while a James Collins effort was blocked on the line by Steve Kinniburgh.
Top scorer Harrad was denied by a full-length save from Ryan Clarke in the 28th minute and had a goal ruled out for offside in added time at the end of the first half.
Chances proved few and far between in the second half, although Oxford showed more in the way of attacking ambition.
However, the introduction of Webster proved an inspired move when the substitute rose to power home a header.
Tom Craddock had a chance to take the tie to a replay but saw his shot deflected wide.