Hypnotherapy is a form of psychotherapy used to create a subconscious change in a subject in the form of new responses, thoughts, attitudes, behaviors, or feelings. It is undertaken with a subject in hypnosis.
Triple agent Ocelot frequently used hypnotherapy in his work. In particular, he and others used hypnotherapy to transplant the mind of Big Boss into a comatose Venom Snake in the 1970s and 1980s on the orders of Zero to act as a decoy.
Decades later, Big Boss would later reveal to Solid Snake that Revolver Ocelot utilized hypnotherapy in the 2000s to transplant the then-deceased Liquid Snake's personality into himself, forming Liquid Ocelot. He did this in order to confuse the Patriots during his attempt to bring them down.