Brentford disrespected Oxford, the Capital One Cup as well as their supporters and were deservedly punished after an embarrassing performance from what was a reserve side.
Marinus Dijkhuizen made 11 changes to the side that drew with Ipswich with Nico Yennaris having made the most appearances for the club with 11.
Liam Sercombe opened the scoring after five minutes after getting the better of Andreas Bjelland and driving the ball past goalkeeper Jack Bonham.
The stopper's nightmare day became worse as his attempted clearance cannoned into Danny Hylton to make it 2-0 before his headed clearance was returned with aplomb from 40 yards by Kemar Roofe on 12 minutes.
Jon Mullins added a fourth soon after half-time as the home side failed to clear their lines.
