Chauncey Wright

All observers not laboring under hallucinations of the senses are agreed, or can be made to agree, about facts of sensible experience, through evidence toward which the intellect is merely passive, and over which the individual will and character have no control.

Sometimes it's even hard to tell the difference between a tic and a compulsion. But while tics stem from an urge in a specific part of the body - either completely unconsciously or through a premonitory sensation that's satisfied only by the tic - OCD bubbles up as conscious thoughts in the mind.