Ordo Templi Orientis (O.T.O.) (Order of the Temple of the East, or the Order of Oriental Templars) is an international fraternal and religious organizationfounded at the beginning of the 20th century. English author and occultist Aleister Crowley has become the most well known member of the order.
Originally it was intended to be modelled after and associated with Freemasonry, but under the leadership of Aleister Crowley, O.T.O. was reorganized around the Law of Thelema as its central religious principle. This Law—expressed as “Do what thou wilt shall be the whole of the Law” and “Love is the law, love under will” —was promulgated in 1904 with the dictation of The Book of the Law.
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 to establish fraternal bonds and impart spiritual and philosophical teachings.
O.T.O. also includes the Ecclesia Gnostica Catholica (EGC) or Gnostic Catholic Church, which is the ecclesiastical arm of the Order. Its central rite, which is public, is called Liber XV, or the Gnostic Mass.