OASIS (Ontologically Anthropocentric Sensory Immersive Simulation) is 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 coevolutionized the entirety of gaming and made up for all of GSS' 10-or-so years of lost revenue they spent creating it. It costs only 25 cents to use and operate, as well as buy, but in-game vending, teleportation, fuel, and almost everything else costs real-world money, which was represented by game money, or "credits." James Halliday hid the Easter Egg competition inside OASIS, and many "gunters" set to finding it.
The universe of OASIS is split into 27 sectors each 10 light hours across, arranged like a Rubik's cube. The sectors have zones that can be any shape and size, and each has different rules/flags. Zones could allow or prevent magic, technology, PvP (Player vs. Player), teleporting, communications, and more. For instance, Ludus, the schooling planet, is a no-PvP zone. The worlds within OASIS can also have different settings. It is described that Ludus is always day, Neonoir is always night, and Archaide's sky never changes with a volcano always on the horizon. Planets can be player built, owned, and colonized, or created by the developers.
Within OASIS, there are NPCs (non-player characters) which a player can interact with. Some will drop experience and items when killed. There are also quests, which a player (or group of players) can take on to get a prize, usually experience, credits and sometimes items. On rare occasions players can receive unique and powerful artifacts from killing the final boss of a quest.
The game had a standard immersion rig given to all new OASIS users, which consisted of a pair of virtual reality goggles and a pair of haptic gloves, 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.