The OASIS was developed by a brilliant billionaire with Asperger's by the name of James Halliday. Halliday invents the OASIS as a place where people can go and interact and learn under assumed identities using Avatars. The OASIS grows, spawning worlds full of games and challenges, but it also has its practical uses for business, news, and education. The most important thing to the protagonist of the story, Wade Watts, is that education in the simulated world is free. Not just free of cost, but free of bullies that made his real-life schooling experience a nightmare.
What sets this story into motion is the death of James Halliday. For it us upon this fateful day that Halliday's avatar Anorak, announces to the world that he has arranged a contest open to everyone within the OASIS to uncover the Easter Egg that is the key to his fortune and power over the OASIS. Halliday programs three keys to three gates within the expansive universe of the OASIS, and he provides the public with the first clue to find the first key. And thus the race for the Egg begins.