In-form Oxford beat League 1 rivals Sunderland 4-2 on penalties after a 1-1 draw at a sold-out Kassam Stadium to reach the quarter-finals of the Carabao Cup.
Rob Hall fired Oxford in front in the 25th minute with an emphatic left-footed shot but Sunderland equalised 12 minutes from time when Marc McNulty turned the ball home from close range following a corner.
James Henry, Anthony Forde, Tariq Fosu and John Mousinho all converted for the home side in the penalty shootout.
However, Will Grigg blasted over for Sunderland, and when McNulty saw his effort saved by Simon Eastwood, United were home and dry and in the last eight of the competition for the first time since the 1987-88 season.
Oxford, winners in 1986, were in a rich vein of form on a 10-match unbeaten run going into this clash.
Both clubs made five changes to the teams in league action last weekend, Oxford beating Rochdale 3-0 and Sunderland losing 1-0 at Shrewsbury.
The Black Cats started strongly with McNulty firing across the face of the goal in the opening minute.
The visitors enjoyed good possession for the first 20 minutes and nearly took the lead when McNulty broke free on the right edge of the box and fired against the outside of the far post.
Immediately the hosts counter-attacked and, after Tariq Fosu was thwarted and the move broke down, the ball fell invitingly for Hall to drive home from 15 yards.
Sunderland missed a glorious chance just before half-time when Max Power turned McNulty's cross wide from just four yards out at the near post.
Karl Robinson's team played with more cohesion after the break and looked more threatening.
Shandon Baptiste, hero of Oxford's thrashing of West Ham in the previous round, sent one effort wide and then, after good work by James Henry, the midfielder was denied by a fine save from Jon McLaughlin.
Sunderland nearly capitalised, though, when Baptiste gave the ball away with a wayward pass, and Aiden McGeady probably should have done better than drill his low long-range shot well wide.
Some sustained Sunderland pressure, following the introduction of Luke O'Nien, had the hosts with their backs to the wall.
Eastwood had to turn a swerving cross onto his own bar, and Henry made a last-ditch defensive clearance for a corner.
An equaliser was on the cards and after McNulty just failed to reach an inviting ball through the middle, he made no mistake from the 78th-minute corner.
The tie remained on a knife-edge until the finish, and it was Oxford who showed the greater composure in the shootout.
Both sides return to League One on Saturday, with Oxford visiting Portsmouth and Sunderland hosting Southend, both at 3pm.