Most obsessions in OCD can be put into one of four main categories, as described in an article in Psychopathology. The first is fear of contamination. The second is related to doubt — for instance, someone doubts they locked their doors despite checking numerous times.
The third type of obsession is taboo-related. These obsessions are often of a sexual, violent, or religious nature. Examples include thoughts of molesting children, disrespecting religious figures, and thoughts of physically hurting others. Anyone can have such thoughts, but generally they are fleeting and can be brushed off. However, someone with OCD finds them very distressing and hard to control. Those with OCD typically have no history of violence toward others, nor do they act on their thoughts (per Psychopathology). But according to a study in the Journal of Obsessive-Compulsive and Related Disorders, obsessions with taboo content tend to be more stigmatized than the other types — thus people who have them are less likely to talk about them.
The fourth type of obsession is related to symmetry and order. Being a “neat freak” does not necessarily mean you have OCD, says psychologist Jon Abramowitz (via the Anxiety Depression Association of America). According to the Diagnostic and Statistical Manual of Mental Disorders Fifth Edition (DSM-5), in order to qualify as a symptom of OCD, the need for tidiness would have to cause extreme distress, get in the way of living an aspect of life, or take an exorbitant amount of time. For instance, someone with OCD may spend a couple of hours arranging their office before being able to sit down to work.