In obsessive-compulsive disorder (OCD), patients have obsessions and compulsions. The obsessions are recurrent thoughts, fears, images, or impulses. The compulsions are recurrent, irresistible actions such as counting, hand washing, or systematically arranging things. People with schizophrenia have hallucinations and suffer from delusions. When there is no identifiable physical cause to explain physical symptoms, a somatoform disorder may be the diagnosis. People who lie, cheat, steal, and have no sense of responsibility exhibit the symptoms of antisocial personality disorder. Impulse control disorders are an inability to resist an impulse to perform an act that is harmful to the individual or others. Substance abuse and chemical dependence fall under this category.

People with obsessive-compulsive disorder often feel the need to perform certain rituals or routines repeatedly. These individuals are often compelled by intrusive thoughts. An individual compelled to perform repeated behavior, such as continually locking and unlocking a door, will often feel distressed by these intrusive thoughts and the need to perform certain behaviors. Someone with obsessive-compulsive disorder cannot simply feel dismissive toward these thoughts, which is part of the distress they experience. While performing these rituals can sometimes cause a temporary sense of relief, this relief does not last for the rest of the day. It is only a transient sense of relief that may come from temporarily satisfying the intrusive thoughts. An individual compelled to perform repeated behavior would have no particular reason to feel content or relaxed.

Trichotillomania is associated with irresistible urges to perform unwanted repetitive behavior, which falls under the obsessive-compulsive spectrum. Although the disorder may cause anxiety to an individual, it does not fit on the anxiety spectrum of behavioral disorders. The autism and bipolar disorder spectrums do not include behaviors or characteristics that are associated with trichotillomania.

Obsessive compulsive disorder, or OCD, is related to a decrease in bioavailable "serotonin" levels. This may be due to alterations serotonin transporter (SERT) function, which increase SERT activity, thus clearing serotonin from the synaptic cleft more rapidly and leading to a shorter duration of effect. While any of the other signaling molecules listed may affected in OCD, serotonin is the only neurotransmitter known to have a direct association with the disorder.

In the context of obsessive compulsive disorder, an obsession refers to "persistent or recurrent involuntary thoughts and/or images which are intrusive and cause marked distress or anxiety." These thoughts often lead to compulsions, or repetitive behavior that an individual feels driven to perform according to rigid rules or rituals.

While it may occur in children under five, obsessive compulsive disorder generally presents either in childhood (age 10-12) or in early adulthood (ages 18-22). Onset after age 25 is less common, and onset in middle age is rare.