The OASIS (Ontologically Anthropocentric Sensory Immersive Simulation) was a MMOSG (massively multiplayer online simulation game) created by James Halliday and Ogden Morrow of Gregarious Simulation Systems (GSS), formerly known as Gregarious Games. It completely revolutionized the entire industry of gaming and made up for all of GSS' 10-or-so lost years that they were programming it. It costs only 25 cents to use and operate, as well as buy, but in-game vending, teleportation, and almost everything else costs real-world money, which was represented by game money, or "credits." James Halliday hid the Easter Egg competition inside the OASIS, and many "gunters" set to finding the egg.
The OASIS is made up of many different planets and worlds, which have different purposes and amenities. The solar systems of the OASIS are split into sectors and zones; some allow magic and/or technology, and others don't allow magic and/or technology to function. Zones come in many different sizes, and they can be PvP enabled or not allow PvP. For instance, Ludus, the schooling planet, is a no-PvP zone. Players aren't allowed to harm each other there. Other planets have quest portals that allow players to get credits, XP, and items, occasionally extremely rare artifacts that have special powers. Only one of each artifact is present in the OASIS.
The OASIS is the primary setting of the book, and not much focus is placed on the outside world until closer to the end.
The game had a standard immersion rig given to all new OASIS users, but high-tech and hacker rigs were constantly being created and upgraded. The most state-of-the-art immersion rig, found in Ogden Morrow's personal home, included a full-body immersion suit inside a "hamster-ball" interior that the user could walk around in to simulate real walking/running, and a form-fitting haptic chair, gloves pair, and visor that gave the user every sensation that their character would feel. The immersion rig at Wade Watts's apartment also featured a "smell tower" which replicated to smells within the OASIS.