AO may stand for:

• Adults Only, contents rated not suitable for people under 18 by Entertainment Software Rating Board
• Auxiliary oiler, US Navy hull classification symbol
• Anarchy Online, a science fiction Massive(ly) multiplayer online role-playing game
• Angola, 2-letter ISO and obsolete NATO country code
• Aosta Valley (Vallée d'Aoste, Valle d'Aosta), abbreviation for the region and province of Italy
• Arctic oscillation
• Atlas Orthogonal
• Australian Airlines, IATA code
• Officer of the Order of Australia, postnominal letters

In addition: The Entertainment Software Rating Board (ESRB) is a self-regulatory organization that applies and enforces ratings, advertising guidelines, and online privacy principles for computer and video games in the United States and Canada (officially adopted by individual provinces 2004-2005). ... The United States Navy (USN) is the branch of the United States armed forces responsible for naval operations. ... The United States Navy uses hull classification symbols (sometimes called hull codes) to identify the types of its ships. ... Anarchy Online is a science fiction MMORPG released in June 2001 by Funcom. ... Science fiction is a form of speculative fiction principally dealing with the impact of imagined science and technology, or both, upon society and persons as individuals. ... MMORPGs (Massively Multiplayer Online Role-Playing Games) follow a client-server model in which players, running the client software, are represented in the game world by an avatar — a graphical representation of the character they play. ... It has been suggested that List of ISO country codes be merged into this article or section. ... This is the list of NATO country codes. ... Aosta Valley (in French Vallée dAoste, in Italian Valle dAosta) is a mountainous region in north-western Italy, the smallest of Italys regions. ... ISO 3166-2 codes for Italy cover 103 provinces. ... The Arctic oscillation (AO) is the dominant pattern of non-seasonal sea-level pressure (SLP) variations north of 20N, and it is characterized by SLP anomalies of one sign in the Arctic and anomalies of opposite sign centered about 37-45N. The North Atlantic oscillation (NAO) is a close relative... The abbreviation ATLAS can stand for one of several things: Abbreviated Test Language for Avionics Systems, a tool for testing military avionics equipment ATLAS experiment, the; a particle detector at CERN ATLAS (simulation), a military exercise simulation system used by the Royal Thai Army Automatically Tuned Linear Algebra Software, an... Australian Airlines is a low-cost airline based in Australia, servicing Australian and Asian destinations. ... The International Air Transport Association is an international trade organization of airlines headquarted in Montreal, Quebec, Canada. ... The Order of Australia is an order of chivalry established by Queen Elizabeth II on February 14, 1975 for the purpose of according recognition to Australian citizens and other persons for achievement or for meritorious service. The Order includes three classes in general and military divisions, in descending order of...

• "Ao" is a Japanese word meaning both 'blue' and 'green'.
• Ao - the people of north east India
• Ao - Tibeto-Burman language of the Ao people
• Ao - the Polynesian god
• Lord Ao - Fictional god in Dungeons & Dragons.

See also Ao is a Japanese word including what Westerners would call, separately, blue and green. ... The Ao are a Naga people of Nagaland in northeast India. ... Ao is a Kuki-Chin-Naga language (of the Tibeto-Burman family) spoken by the Ao of Nagaland in northeast India. ... In Polynesian mythology, Ao (light) is the god of the clouds and the ancestor of the Maori. ... Lord Ao (ay-oh) is the Overgod of the fictitious, magical world of Abeir-Toril. ... Players Handbook for D&D Dungeons & Dragons (abbreviated as D&D, DnD, or AD&D for the advanced edition) is a fantasy tabletop role-playing game (RPG) published by Wizards of the Coast. ...

• A0 (A-zero)
• .ao (the ccTLD for Angola).