Complex sentence: It is often associated with obsessive--compulsive disorder, in which a sufferer will repeatedly (usually unsuccessfully) attempt to prevent or "neutralize" intrusive distressing thoughts centered around one or more obsessions.
Split into simpler sentences: It is often associated with obsessive--compulsive disorder,. OCD is when a person will repeatedly (usually unsuccessfully) attempt to prevent or "neutralize" intrusive distressing thoughts centered around one or more obsessions.