The Hammers are due to become anchor tenants at the stadium in 2016 but Hearn claims that is "not a done deal" because of his ongoing judicial review and legal action against the Premier League.
Hearn wants his club to be able to ground-share with West Ham and use just the lower tier for Orient's matches. Hearn, giving evidence to the House of Lords committee on Olympic and Paralympic legacy on Wednesday, said: "This is not a done deal. West Ham have an agreement but I have a judicial review outstanding and litigation with the Premier League that West Ham even moving would be a breach of Premier League rules.
"We have ended up in situation where we have gifted ?500million of tax-payers' money to a Premier League club that has a turnover of more than ?100million. It's a wonderful gift but if I was an Arsenal fan I would wonder why we bothered paying to build a new stadium.
"This is state sponsorship beyond my wildest dreams. In effect it's rent free as they have ability to develop Upton Park."
West Ham's vice-chairman Karren Brady insisted that the club was not getting the stadium on the cheap and that the deal was good value to the tax-payer.
Brady was asked to respond to a claim by former sports minister Richard Caborn that West Ham was getting the ?600million stadium for just ?15million plus ?2million a year rent with the tax payer picking up the tab.
Brady defended the deal however, saying: "The alternative scenario does not bear thinking about.
"West Ham is not getting a free stadium. The stadium was built for the Olympics and what are we going to do with it? Across the world stadiums that are not used, die.
"Without anchor tenants the cost would be huge to the tax payer."