Oxford extended their unbeaten run to nine games thanks to a comfortable victory at struggling AFC Wimbledon.
United led when Alfie Potter set up Sean Rigg to hammer home and roles were then reversed as Rigg crossed for Potter to score a rare header.
The hosts missed chances at the start of the second half and were made to pay when Tom Craddock pounced on a poor Dons header to fire home.
The U's ended with 10 men as James Constable saw red for an alleged elbow.
United have confirmed they will appeal against the red card and the four-game ban that will come with it.