Ordo Templi Orientis (O.T.O.) ('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. English author and occultist Aleister Crowley is the best-known and most influential member of the order.
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. One of the major features and core teachings of the organization is its practice of sex magic.
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.
According to Crowley, O.T.O. members should rarely come into contact with one another. "Even after affiliation, you would not meet anyone unless it were necessary for you to work in cooperation with them. I am afraid you have still got the idea that the Great Work is a tea-party. Contact with other students only means that you criticize their hats, and then their morals, and I am not going to encourage this. Your work is not anybody else’s, and undirected chatter is the worst poisonous element in a humane society." [Magick Without Tears]
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.