The Hammers said the 18-year-old turned down interest from clubs including Manchester United, Manchester City and Arsenal.
Oxford was the second youngest player to start a Premier League match when he made his competitive debut against Arsenal, aged 16, at the start of last season and starred in a 2-0 win at the Emirates Stadium.
“I just want to kick on now,” he said.
“There was a lot of speculation about my future but I am happy it has been done and I can get back to football.”
West Ham boss Slaven Bilic expects big things from the teenager, who has missed the last six weeks with an ankle injury but is expected to resume training next week.
“I really believe he has got everything,” said Bilic. “On paper he has all the abilities to become a world-class player and it is now up to him to train hard and for us to manage him well for him to fulfil it.”
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