The letters O.T.O. stand for Ordo Templi Orientis, the Order of Oriental Templars, or Order of the Temple of the East. O.T.O. is dedicated to the high purpose of securing the Liberty of the Individual and his or her advancement in Light, Wisdom, Understanding, Knowledge and Power through Beauty, Courage and Wit, on the Foundation of Universal Brotherhood.
Ordo Templi Orientis is the first of the great Old Æon orders to accept The Book of the Law, received by Aleister Crowley in Cairo, Egypt in 1904. This book proclaims a New Æon in human thought, culture and religion. The Æon arises from a single supreme injunction: the Law of Thelema, which is Do what thou wilt.
If you are interested in becoming a member of Ordo Templi Orientis, see the Membership page for more information. This FAQ from the U.S. Grand Lodge answers many common questions about O.T.O. and Thelema.