Excerpt from a letter by Crowley to Karl Germer

The difference between A A and the O.T.O. is very clear and simple. The A A is a sempiternal institution and entirely secret. There is no communication between its members. Theoretically, a member knows only the superior who introduced him, and any person whom he himself has introduced. The Order is run on purely spiritual lines.
The Object of membership is also entirely simple. The first objective is the Knowledge and Conversation of the Holy Guardian Angel. The next objective, omitting considerations for the present of the 6 = 5, and the 7 = 4 degrees, is the crossing of the Abyss, and the attainment of mastership of the Temple. This is described very fully especially in Liber 418...

A.'.A.'. :..it stands by itself as a body similar to Freemasonry, but involving acceptance of a social and economic system which is intended to put the world on its feet. There is also, of course, the secret of the IXth which is, so to say, the weapon which they may use to further these purposes.
To show you the difference, Theodor Reuss was Supreme Head of the O.T.O., but was not even probationer of the A A.