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Ordo Templi Orientis (O.T.O.; lit. 'Order of the Temple of the East' or 'Order of Oriental Templars') is an occult initiatory organization founded at the beginning of the 20th century. The origins of the O.T.O. can be traced back to the German-speaking occultists Carl Kellner, Heinrich Klein, Franz Hartmann and Theodor Reuss. Later, the O.T.O. was significantly shaped by the English author and occultist Aleister Crowley.
After Crowley's death in 1947, four main branches of the O.T.O. have claimed exclusive descent from the original organization and primacy over the other ones. The most important and visible of these is the Caliphate O.T.O., incorporated by Crowley's student Grady McMurtry in 1979.
Originally it was intended to be modeled after and associated with European Freemasonry, such as Masonic Templar organizations, but under the leadership of Aleister Crowley, O.T.O. was reorganized around Crowley's Thelema as its central religious principle.
Similar to many secret societies, O.T.O. membership is based on an initiatory system with a series of degree ceremonies that use ritual drama.
The O.T.O. includes the Ecclesia Gnostica Catholica (EGC) or Gnostic Catholic Church. Its central rite, which is public, is Liber XV, The Gnostic Mass.