Exposure and Response Prevention (ERP) is the only evidence-based therapy for OCD. It has been extensively researched and has been shown to significantly decrease symptoms in 70% of all cases.
Exposure is the process of gradually exposing yourself to feared situations, thoughts, and images in order to provoke anxiety. Response Prevention is the voluntary blocking of any type of compulsive behavior that would normally decrease the anxiety associated with your obsession.
ERP involves breaking the connections or associations that maintain your OCD. There are two different types of associations that trigger and (through reinforcement) maintain or exacerbate your OCD. The first type of association is created when a certain thought, situation, or image provokes fear or anxiety. When this happens repeatedly, a kind of conditioning occurs and you come to experience anxiety automatically as a result of these specific experiences. Consequently, you will likely begin to avoid these triggers whenever you can. The second type of association is made as compulsive behavior comes to be associated with feeling better and less anxious. As a result, you will tend to engage in compulsions whenever your anxiety is triggered with the expectation of decreasing it.
As you continue to avoid triggers and engage in compulsive behaviors, your OCD symptoms (both the obsessions and compulsions) will likely increase. Although your compulsions may start out being fairly effective at reducing your anxiety, over time they will become more time and energy consuming. You will need to do more and more of them just to feel better. Engaging in compulsions also causes your obsessions to generalize, meaning increasingly more thoughts, images, and situations will become anxiety provoking. More fears, in turn, will lead to more compulsive and avoidance behaviors, spiraling your OCD out of control.
The goal of ERP is to stop this cycle by breaking these associations and creating more functional ones. Repeatedly exposing yourself to your fears, or to whatever provokes your fear, will ultimately break the connection between your obsession and the increased anxiety you experience. This happens through the process of habituation. Habituation simply means you become desensitized or get used to having the obsession and/or doing whatever it is that triggers your obsession so that you no longer experience much, if any, anxiety as a result. Practicing response prevention ultimately breaks the connection between compulsive behavior and the reduction of your anxiety. Not allowing yourself to give in to any compulsions allows your anxiety to come down on its own with just the passing of time. As you gain more experience with and more confidence in this process, you will experience less anxiety and, therefore, less of an urge to engage in compulsive behavior.