The League One hosts made six changes to the side who beat Lincoln City 6-0 at the weekend, with George Thorne handed a first start for the U’s.
Hammers boss Manuel Pellegrini named a strong XI which included winger Nathan Holland, who was making his first start for the club, while Arthur Masuaku returned following suspension.
Cameron Brannagan and Anthony Forde were both unable to make the most of early opportunities for the hosts, while 26 minutes in former Irons forward Robert Hall was unlucky to see a free-kick stay out as it struck the angle of post and crossbar.
The visitors were finding it hard to get into the game, although just past the half-hour mark Jack Wilshere forced a save from home keeper Simon Eastwood.
Early in the second half Hammers keeper Roberto sliced a routine clearance narrowly wide of his own goal and the Spaniard was beaten soon afterwards as the hosts hit the front.
Mark Sykes delivered the ball into the area and when it eventually fell to Elliott Moore, he showed good composure to find the bottom corner of the net with a right-foot shot.
Falling behind prompted the Premier League side to throw on Sebastien Haller, while the U’s went in search of a second goal, with Roberto making a fine save to deny Jamie Mackie.
Brannagan and Hall both threatened as Oxford continued to press and they got their reward 19 minutes from time as Sykes crossed from the right and Matty Taylor arrived to turn the ball home and make the score 2-0.
There was still time for Oxford to put further gloss on the scoreline as Tariqe Fosu-Henry raced clear, rounded Roberto and netted with ease to compound the Hammers’ misery.
And the final ignominy for the visitors came right at the death as Shandon Baptiste added a fourth goal for the hosts deep into stoppage time.
Carabao Cup draw for the fourth round:
Chelsea play Manchester United, Liverpool host Arsena while Colchester travel to Crawley in fourth round of the Carabao Cup. Ties played w/c 28 October
Everton v Watford Aston Villa v Wolves Man City v Southampton Buron v Leciester Crawley v Colchester Chelsea v Man Utd Oxford v Sunderland Liverpool v Arsenal
Oxford United 4-0 West Ham
The League One hosts made six changes to the side who beat Lincoln City 6-0 at the weekend, with George Thorne handed a first start for the U’s.
Hammers boss Manuel Pellegrini named a strong XI which included winger Nathan Holland, who was making his first start for the club, while Arthur Masuaku returned following suspension.
Cameron Brannagan and Anthony Forde were both unable to make the most of early opportunities for the hosts, while 26 minutes in former Irons forward Robert Hall was unlucky to see a free-kick stay out as it struck the angle of post and crossbar.
The visitors were finding it hard to get into the game, although just past the half-hour mark Jack Wilshere forced a save from home keeper Simon Eastwood.
Early in the second half Hammers keeper Roberto sliced a routine clearance narrowly wide of his own goal and the Spaniard was beaten soon afterwards as the hosts hit the front.
Mark Sykes delivered the ball into the area and when it eventually fell to Elliott Moore, he showed good composure to find the bottom corner of the net with a right-foot shot.
Falling behind prompted the Premier League side to throw on Sebastien Haller, while the U’s went in search of a second goal, with Roberto making a fine save to deny Jamie Mackie.
Brannagan and Hall both threatened as Oxford continued to press and they got their reward 19 minutes from time as Sykes crossed from the right and Matty Taylor arrived to turn the ball home and make the score 2-0.
There was still time for Oxford to put further gloss on the scoreline as Tariqe Fosu-Henry raced clear, rounded Roberto and netted with ease to compound the Hammers’ misery.
And the final ignominy for the visitors came right at the death as Shandon Baptiste added a fourth goal for the hosts deep into stoppage time.
Carabao Cup draw for the fourth round:
Chelsea play Manchester United, Liverpool host Arsena while Colchester travel to Crawley in fourth round of the Carabao Cup.
Ties played w/c 28 October
Everton v Watford
Aston Villa v Wolves
Man City v Southampton
Buron v Leciester
Crawley v Colchester
Chelsea v Man Utd
Oxford v Sunderland
Liverpool v Arsenal