Operating Thetan, abbreviated "OT", refers to either a spiritual state, someone who has such attained state or an action.

The definition of OT, according to tape 6 of the 1st Melbourne Advanced Clinical Course (at 2m12s), given on 11 November 1959, is "willing and knowing cause over life, matter, energy, space and time, which would of course include postulates, considerations and thetans."