|Austin celebrates after opening the scoring|
It was almost 3-0 when Byram was again involved, feeding an advancing Diouf, but Ryan Clarke got a hand on the strike as Diouf looked to slip it back across him.
United also started the second half on the front foot and it took some desperate defending to prevent a third goal, although there were claims from Leeds for a foul inside the six-yard box.
Pearce had an effort blocked as United looked to kill the tie off during the early stages of the second period. Austin also ratted the crossbar with a thunderous strike from around 40 yards out.
It was a night of spectacular strikes and Byram was next to try his luck when he sent a volley over the bar after great work by Diouf in teeing him up.
At the other end, Adam Drury cleared the danger when Simon Heslop was closing in on a low centre as Oxford mounted a rare raid. United were soon back on the attack, though, and Michael Brown had a shot blocked.
A third goal did come on 73 minutes when Diouf delivered a cross from the right and Tom Lees powered a header past Clarke in the Oxford goal to seal the tie.
The visitors did look to salvage some pride, Capaldi sending a shot wide, but they were always huffing and puffing against a Leeds side who looked well in control and could have had more goals.