Obsessive Compulsive Disorder is an anxiety disorder, first and foremost.  The essential features of OCD are recurrent obsessions (thoughts) that create an awareness of alarm or threat.  Persons typically engage in some avoidance or escape response in reaction to the obsessive threat. Obsessions take the form of obsessing about being anxious, or a perceived threat of physical harm to oneself or others or, in some cases, more of a metaphysical or spiritual threat to oneself or others.

Compulsions can be from feeling anxious, heightened responses to people or animals, to writing lists of things to do to continual washing.

How Hypnotherapy can work;  Hypnotherapy can be used in a very direct way to remove the thought (obsession) and focus on eliminating or controlling the obsession.  Severe OCD can take several sessions but typically some results are seen after the first session and at the most 3 or 4 sessions are all that are needed.