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. 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.