Obsession - A persistently recurring idea, compelling and difficult to put out of mind.
Obsessive Compulsive - In obsessive compulsive neurosis the patient feels compelled to carry out the persistently recurring idea. This can take many forms, such as excessive hand washing (sometimes the patient feeling the need to wash hands over 100 times per day) or excessive checking of door locks or clothing etc. Treatable with hypnosis.
Oneirosis - An early term for a stage of light hypnosis, similar to hypnogogic state and characterized by visual imagery. From the Greek word oneiros, meaning dreams.
Operator - A name occasionally given to the hypnotist.
Oedipus Complex - A term coined by Sigmund Freud (from a Greek Myth) to describe the complex formed in young males through a form of sexual attraction to the mother, causing jealousy of the father and resulting in a feeling of conflict and guilt. In the Greek myth Oedipus killed his father and unknowingly married his mother.
Onomatomania - Obsession with words, especially the sound of words. Sometimes attaching personal meanings to words apart from their usual definition.
One Trial Learning - A single occurrence or event that has such a powerful impact on the individual that it modifies their behaviour from then onwards. For example, a person becoming violently ill through drinking too much whisky might be unable thereafter to drink whisky again.
Operant Conditioning - A form of learning identified in Behaviourism (B. F. Skinner) in which a person's behaviour (or any animal's for that matter) is modified by positive or negative reinforcement (ie., praise or punishment).
Orgasmaphobia - Irrational fear of orgasm. Typically that the experience would be so intense it would cause the sufferer to expire.
Overcompensation - Conscious or unconscious behaviour designed to make amends for (or disguise) some (real or imagined) short coming.