Karl Robinson masterminded another upset in the Carabao Cup as Oxford reached the fourth round for the first time in 22 years following a 4-0 win over West Ham at the Kassam Stadium.
Second-half goals by Elliott Moore, Matty Taylor, Tariqe Fosu and Shandon Baptiste gave the U's a deserved victory.
Robinson, who led MK Dons to a 4-0 win over Manchester United in this competition five years ago, watched his current side put on a show, but it was a bitterly disappointing night for Manuel Pellegrini's men.
Days after a victory of their own against the Red Devils, the Hammers boss made nine changes and handed a full debut to academy graduate Nathan Holland.
The 21-year-old almost made the perfect start too, when he intercepted Rob Dickie's poor pass and linked up with Albian Ajeti, but Simon Eastwood blocked his low effort with the tie only two minutes old.
West Ham supporters were making plenty of noise during the opening exchanges, including mocking Tottenham, who were dumped out of the competition on penalties at Colchester on Tuesday.
Oxford were out to be the team to cause an upset this time and despite resting Saturday's hat-trick hero Fosu, they threatened twice before quarter of an hour had been played.
Rob Hall, against his former club, saw an effort deflect into the path of Cameron Brannagan in the 13th minute, but the former Liverpool youngster scuffed wide from 10 yards.
Two minutes later and West Ham's back four were cut open again as Baptiste sent Anthony Forde away, although he could only pull his shot wide of the post.
Any momentum built up by Oxford was halted not long after when George Thorne, on loan from Derby, injured his left arm and was replaced by Mark Sykes in the 23rd minute.
Impressively, Robinson's side quickly got back into their stride after the substitution and hit the crossbar with 26 played when Hall's free kick almost crept in.
West Ham responded with the lively Holland testing Eastwood from a tight angle on the left before Moore cleared the loose ball.
Jack Wilshere tried his luck from range in the 32nd minute, yet it was comfortable for the U's goalkeeper, although there was clearly an increase in urgency from the Premier League side.
Robert Snodgrass was denied a shot on goal by Rob Dickie early in the second period before the hosts' confidence increased again when Roberto sliced a clearance behind for a corner.
West Ham survived that set-piece and the next, but Robinson's team kept the ball alive from Forde's free-kick and substitute Sykes crossed in for centre-back Moore, who controlled and fired into the bottom corner from eight yards with 55 on the clock.
A cup shock was firmly on the cards and 22-year-old Sykes was giving Arthur Masuaku plenty of problems and almost created a second for Oxford two minutes later when Mackie flicked goalwards, but Roberto produced a brilliant stop to save the Hammers.
It was one-way traffic now, with the U's eager for a second and Pellegrini reacted by introducing Sebastien Haller and Felipe Anderson, but it failed to have the desired effect.
Backed by 9,000 home supporters, Oxford increased their lead with 19 minutes left.
90+2' GOOOOOOAAAAAAAAALLLLLLLLL BAPTIIIIIISSSSSTTTEEEEE #OUFC 4-0 #WHUFC pic.twitter.com/l0sLND3v4N — Oxford United FC (@OUFCOfficial) September 25, 2019
Glenavon's finest Sykes was involved, allowed too much space on the right and he crossed in for fellow substitute Taylor to tap home at the back post.
Robinson had only introduced the Bristol City loanee two minutes early and West Ham captain Mark Noble entered the pitch soon after, but this night belonged to Oxford and the hosts' third substitute added another.
Fosu, fresh from a treble at Lincoln on Saturday, slipped past Masuaku by the halfway line, rounded Roberto and slotted home with four minutes left.
There was still time for one more goal when Baptiste dribbled into the area and side-footed into the bottom corner in stoppage time to send Oxford into the fourth round for the first time since 1997.
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