Match ends, Oxford United 4, West Ham United 0.
West Ham manager Manuel Pellegrini took responsibility as his side were bundled out of the Carabao Cup in humiliating fashion by League One Oxford United.
The Hammers arrived at the Kassam Stadium on the back of an outstanding win over Manchester United to reach the heights of fifth in the Premier League, but Karl Robinson's side superbly outplayed and over-powered their illustrious opponents, who ended a complete shambles.
Oxford, currently 12th in League One, were high on confidence after a record-breaking 6-0 win at Lincoln City at the weekend and this was a famous and richly merited triumph.
"The whole team didn't play well. Not only did we concede four goals, but we didn't create too many chances," Pellegrini said.
"We missed too many passes from the beginning and the responsibility is first on me because I picked the players, and second on the team that didn't compete.
"It's easy to say that we played very badly, but Oxford did everything they needed to win this game. They played with a lot of motivation, with desire and we didn't play well."
Both sides fielded much-changed sides and the only surprise was West Ham actually survived for so long before conceding after 55 minutes when Oxford central defender Elliott Moore shot across Roberto.
Oxford, who missed clear chances in the first half through Cameron Brannagan and Anthony Forde, almost increased their lead immediately as Roberto saved brilliantly from Jamie Mackie.
But there was no escape for West Ham as substitute Matty Taylor turned in Mark Sykes' cross with 19 minutes left.
Substitute Tariqe Fosu, a hat-trick hero at Lincoln City, raced clear from the halfway line to score with great composure after 84 minutes to extinguish any hopes of a West Ham comeback.
The agony was not over yet for West Ham boss Pellegrini and his abject side as Shandon Baptiste deservedly capped a man-of-the-match showing with a classy fourth.
Oxford host fellow League One side Sunderland, who won 1-0 at Sheffield United, in the fourth round in the week commencing 28 October.
"Looking at the scoreboard - 4-0 against West Ham and a very strong West Ham - this will probably go down as one of the biggest results at the Kassam in recent years," Oxford boss Robinson said.
Humiliation for West Ham and Pellegrini
The EFL Cup represented a realistic opportunity for West Ham to win a trophy, but Pellegrini risked this shock by making nine changes, leaving out danger men Sebastien Haller and Felipe Anderson.
He paid the price as the Hammers produced a desperate display, much to the annoyance of the fans who packed one corner of this three-sided stadium.
West Ham were lethargic, off the pace and apparently complacent as they were hustled out of their stride as the track-suited Pellegrini failed to inspire his team.
He kept Haller back until they were a goal down but by then the momentum was flowing inexorably in the direction of Oxford, who should have inflicted even heavier punishment as they ran riot towards the end.
Jack Wilshere wasted an opportunity to stake a claim as he was over-run by the energy of Baptiste. Wilshere looked a spent force in contrast.
It was reminiscent of West Ham's loss at League One AFC Wimbledon in the FA Cup fourth round last season as they lacked heart and stomach for the fight, surrendering without suggesting for one moment they would get back into this game once Oxford went ahead.
West Ham have shown signs of stability and improvement in the Premier League, and this was an altered line-up, but their performance was inexcusable and all the plaudits must go to the underdogs.
"I thought it was a poor performance from everyone," said Hammers captain Zabaleta.
"We feel sorry about the performance tonight and sorry to the away fans who came to the game. It was just a bad night."
Oxford's glory night
This competition gave Oxford the greatest day in their history when they beat QPR at Wembley in 1986 - and this is a night that will also live long in the memory of the jubilant fans.
West Ham made changes but, for context, Oxford manager Robinson also made six changes and his team dealt much better with those alterations.
Oxford were in command from the first whistle, sensing immediately that West Ham were not in the right frame of mind to face a lower league opponent determined to inflict a shock.
Robinson may have feared the first-half misses from Brannagan and Forde may haunt them, but they won at a canter and it would not have been unfair had they enjoyed an even greater victory margin.
Oxford have now scored 10 goals without reply in their past two games and this scoreline was a more than accurate reflection of the gulf between the two sides.
Baptiste was outstanding in midfield, the veteran Mackie was a threat throughout and it is huge credit to Robinson and his players that they never took a backward step once Moore put them ahead 10 minutes after the break.
Oxford continued to be bold and go in search of goals, and the celebrations on and off the pitch at the final whistle were fully deserved.
This was a night of shame for West Ham, but it would be an insult and injustice to downgrade the quality of Oxford's performance that brought this outstanding victory.
"We had the belief in ourselves that we could get a result," Robinson said. "I'm over the moon for the fans and the players.
"It's about the players, about the fans and about the community of Oxfordshire and the big thing for me now is the people who came here for the first time in a long time, that they buy a ticket and come on Saturday [for the league game against Gillingham]."
- 5MooreBooked at 66mins
- 7HallSubstituted forFosu-Henryat 79'minutes
- 22ThorneSubstituted forSykesat 23'minutes
- 19MackieSubstituted forTaylorat 70'minutes
- 15C Sánchez
- 11SnodgrassBooked at 68minsSubstituted forNobleat 74'minutes
- 19WilshereSubstituted forFelipe Andersonat 66'minutes
- 37HollandSubstituted forHallerat 57'minutes
- 8Felipe Anderson
- 20Soares Cardoso
- Robert Jones
- Shots on Target
Second Half ends, Oxford United 4, West Ham United 0.
Corner, West Ham United. Conceded by Shandon Baptiste.
Attempt blocked. Pablo Fornals (West Ham United) left footed shot from outside the box is blocked.
Felipe Anderson (West Ham United) wins a free kick on the left wing.
Foul by Sam Long (Oxford United).
Pablo Zabaleta (West Ham United) wins a free kick on the right wing.
Foul by Tariqe Fosu-Henry (Oxford United).
Goal! Oxford United 4, West Ham United 0. Shandon Baptiste (Oxford United) right footed shot from the centre of the box to the bottom right corner. Assisted by Matty Taylor.
Attempt saved. Anthony Forde (Oxford United) right footed shot from outside the box is saved in the top left corner. Assisted by Tariqe Fosu-Henry.
Attempt saved. Pablo Fornals (West Ham United) header from the centre of the box is saved in the centre of the goal. Assisted by Albian Ajeti.
Offside, Oxford United. Sam Long tries a through ball, but Josh Ruffels is caught offside.
Foul by Pablo Zabaleta (West Ham United).
Tariqe Fosu-Henry (Oxford United) wins a free kick on the left wing.
Attempt saved. Sam Long (Oxford United) header from the left side of the six yard box is saved in the top left corner. Assisted by Robert Dickie with a headed pass.
Corner, Oxford United. Conceded by Pablo Zabaleta.
Attempt missed. Felipe Anderson (West Ham United) right footed shot from outside the box is close, but misses the top right corner. Assisted by Arthur Masuaku.
Arthur Masuaku (West Ham United) wins a free kick in the defensive half.
Foul by Mark Sykes (Oxford United).
Foul by Sébastien Haller (West Ham United).