The club confirmed the starlet's passing at the age of 15 in a statement on social media.
In a message shared to X, the Hammers said: "It is with deep and profound sadness that West Ham United confirm the tragic passing of our U15s Academy goalkeeper Oscar Fairs, following his brave battle with cancer.
"Rest in peace, brave Oscar."
Following Fairs' passing, all West Ham academy fixtures scheduled for this weekend have been postponed out of respect.
They include the under-21 side's Premier League 2 clash with Stoke.
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Sporting director Mark Noble also paid tribute to Fairs, who was part of the club's under-15 team, in a separate statement.
In a message released on the club website, the former captain said: "On behalf of everyone at West Ham United, it is with deep and profound sadness that I confirm the tragic passing of our U15s Academy goalkeeper Oscar Fairs, following his brave battle with cancer.
"Oscar was adored by everyone at the Academy - not only was he a great goalkeeper, he was a true Hammer and a fantastic young person, who will be deeply missed by everyone who had the pleasure to know him.
"I have wonderful memories of Oscar playing in my garden - Lenny and his teammates all loved him. He was a friendly, happy, well-mannered and polite young man, who had such a bright future ahead of him, and it is just so unimaginably devastating that he has been taken from his family and friends at this age.
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"As a mark of respect, all scheduled Academy fixtures this weekend have been postponed, including this evening's PL2 Under-21 match against Stoke City.
"In due course, the Club will also share information on our plans to pay tribute to Oscar, and we will ensure that his name is never forgotten at West Ham United.
"The thoughts and sincere condolences of everyone at the Club are with Oscar's parents, Natalie and Russell, and his brother Harry, and we kindly ask that the family's privacy is respected at this extremely difficult time.
"Rest in peace, brave Oscar."
Tributes have since flooded in from the Premier League as well as clubs including Fulham and Plymouth.
West Ham Women also wrote: "Rest in peace, Oscar. Our heartfelt thoughts and deepest condolences go out to his family and friends."