"OT" is an acronym for Operating Thetan. An OT is, properly, a being who can operate in the physical universe independent of a body. Such a being would be able to move MEST objects, communicate easily with others via thought transference, and so forth. These concepts are familiar in science fiction
The OT levels are those sections of the practices of Scientology which lead the person who is already Clear and has done the lower bridge with the Grades further up toward spiritual freedom and increased abilities. The abilities gained from doing these levels are assumed to be leading one towards the state of OT as above, but do not deliver the state of being able to operate without a body. A person who has completed one or more of these levels is often called "An OT", but that is something of a misnomer, and should not be confused with the state mentioned above. If there is a technology at this time on Planet Earth to achieve this state, it is not generally known.