Common compulsions include excessive hand washing, cleaning, counting, ordering, repeating, avoiding triggers, hoarding, neutralizing, seeking assurance, praying and checking things.

While this is sometimes referred to as primarily obsessional obsessive–compulsive disorder (Pure O), it is also considered a misnomer due to associated mental compulsions and reassurance seeking behaviors that are consistent with OCD.

[24] Some patients fail to improve after taking the maximum tolerated dose of multiple SSRIs for at least two months; these cases qualify as treatment-resistant and can require second-line treatment such as clomipramine or atypical antipsychotic augmentation.

[4][5][27][28] While SSRIs continue to be first-line, recent data for treatment-resistant OCD supports adjunctive use of neuroleptic medications, deep brain stimulation and neurosurgical ablation.

A relatively vague obsession could involve a general sense of disarray or tension, accompanied by a belief that life cannot proceed as normal while the imbalance remains.

[50][51] Some people with OCD experience sexual obsessions that may involve intrusive thoughts or images of "kissing, touching, fondling, oral sex, anal sex, intercourse, incest and rape" with "strangers, acquaintances, parents, children, family members, friends, coworkers, animals and religious figures" and can include heterosexual or homosexual contact with people of any age.

For example, someone who engages in compulsive hoarding might be inclined to treat inorganic matter as if it had the sentience or rights of living organisms, despite accepting that such behavior is irrational on an intellectual level.

[60] Furthermore, compulsions are different from tics (such as touching, tapping, rubbing or blinking) and stereotyped movements (such as head banging, body rocking or self-biting), which are usually not as complex and not precipitated by obsessions.

[9] In addition to experiencing the anxiety and fear that typically accompanies OCD, affected individuals may spend hours performing compulsions every day.

The absence of insight altogether, in which the individual is completely convinced that their beliefs are true, is also identified as a delusional thought pattern and occurs in about 4% of people with OCD.

[75] Furthermore, severe and frequent overvalued ideas are considered similar to idealized values, which are so rigidly held by, and so important to affected individuals, that they end up becoming a defining identity.

[21] Some people with OCD have areas of unusually high activity in their brain or low levels of the chemical serotonin,[129] which is a neurotransmitter that some nerve cells use to communicate with each other,[130] and is thought to be involved in regulating many functions, influencing emotions, mood, memory and sleep.

[136][137] The CANS and PANS hypotheses include different possible mechanisms underlying acute-onset neuropsychiatric conditions, but do not exclude GABHS infections as a cause in a subset of individuals.

Symptom-specific neuroimaging abnormalities include the hyperactivity of caudate and ACC in checking rituals, while finding increased activity of cortical and cerebellar regions in contamination-related symptoms.

This is supported by the observation that those with OCD demonstrate decreased activation of the ventral striatum when anticipating monetary reward, as well as increased functional connectivity between the VS and the OFC.

Studies of peripheral markers of serotonin, as well as challenges with proserotonergic compounds have yielded inconsistent results, including evidence pointing towards basal hyperactivity of serotonergic systems.

[157] Orbitofrontal cortex overactivity is attenuated in people who have successfully responded to SSRI medication, a result believed to be caused by increased stimulation of serotonin receptors 5-HT2A and 5-HT2C.

[17] In regards to diagnosing, the health professional also looks to make sure that the signs of obsessions and compulsions are not the results of any drugs, prescription or recreational, that the patient may be taking.

OCPD, on the other hand, is egosyntonic, marked by the person's acceptance that the characteristics and behaviors displayed as a result are compatible with their self-image, or are otherwise appropriate, correct or reasonable.

[210] In the United States, the Food and Drug Administration (FDA) approved deep brain stimulation for the treatment of OCD under a humanitarian device exemption, requiring that the procedure be performed only in a hospital with special qualifications to do so.

[211] In the United States, psychosurgery for OCD is a treatment of last resort and will not be performed until the person has failed several attempts at medication (at the full dosage) with augmentation, and many months of intensive cognitive behavioral therapy with exposure and ritual/response prevention.

[93][95] A 2024 systeramitc review of the literature found that combining ERP therapy with selective serotonin reuptake inhibitors can enhance treatment outcomes compared to using SSRIs alone.

"[227]: 212  The Cloud of Unknowing, a Christian mystical text from the late 14th century, recommends dealing with recurring obsessions by attempting to ignore them, and, if that fails, to "cower under them like a poor wretch and a coward overcome in battle, and reckon it to be a waste of your time for you to strive any longer against them", a technique now known as emotional flooding.

[227]: 213 Abu Zayd Al-Balkhi, the 9th century Islamic polymath, was likely the first to classify OCD into different types and pioneer cognitive behavioral therapy, in a fashion unique to his era and which was not popular in Greek medicine.

"[227]: 213  In 1584, a woman from Kent, England, named Mrs. Davie, described by a justice of the peace as "a good wife", was nearly burned at the stake after she confessed that she experienced constant, unwanted urges to murder her family.

[233] The success of ERP clinically and scientifically has been summarized as "spectacular" by prominent OCD researcher Stanley Rachman decades following Meyer's creation of the method.

Lanzer's illness was characterised most famously by a pattern of distressing intrusive thoughts in which he feared that his father or a female friend would be subjected to a purported Chinese method of torture in which rats would be encouraged to gnaw their way out of a victim's body by a hot poker.

"[246] In a documentary released in 2023, David Beckham shared details about his compelling cleaning rituals, need for symmetry in the fridge and the impact of OCD on his life.

[248] Compassionate and accurate literary and on-screen depictions may help counteract the potential stigma associated with an OCD diagnosis and lead to increased public awareness, understanding and sympathy for such disorders.

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