Ordo Templi Orientis translates to something like "Order of the Temple of the East".
The O.T.O. was conceived in 1895 by Karl Kellner, a high-grade Mason and industrial chemist. In the East, Kellner met three adepts who trained him in magickal practice. Kellner was assisted in forming the Order by people with whom he had worked before, Franz Hartmann, Heinrich Klein and Theodore Reuss. In 1902, the Order was first proclaimed in Reuss' Masonic publication, 'Oriflamme'. Following Kellner's demise, Reuss became the Outer Head (O.H.O.) of the Order.
The purpose of the O.T.O. is to attempt to enlighten its members by using modern magic based on ancient techniques. They prepare the aspirant to receive the secrets of lost mysteries and finally, the One Hermetic Truth.
Some favourite subjects of study are Hermetic magic, Kabbalah, Enochian magic, and sex magic.
Source: 'FAQ: What is OTO?' by Tyagi Mordred Nagasiva, containing the quotes of Frater Superior Merlin Peregrinus X Degree, Past Grand Master Albert Karl Theodor Reuss.