Obsessive–compulsive disorder (OCD) is a mental condition in which a person feels the need to perform certain routines repeatedly (called "compulsions"), or has certain thoughts repeatedly (called "obsessions"). The sufferer is unable to control either the thoughts or activities for more than a short period of time.

The cause of the disorder has not yet been established.

Data […] suggests that three brain areas are involved with OCD: the orbitofrontal cortex (OFC), the anterior cingulate cortex (ACC), and the head of the caudate nucleus. Several studies have found that in patients with OCD, these areas: (1) are hyperactive at rest relative to healthy control; (2) become increasingly active with symptom provocation; and (3) no longer exhibit hyperactivity following successful treatment with SRI pharmacotherapy or cognitive-based therapy. This understanding is frequently cited as evidence that abnormality in these neuroanatomical regions may cause OCD.

Source : Wikipedia

Note that the current first-line treatment for managing the disorder is usually the prescription of SSRI drugs - notably Fluoxetineplugin-autotooltip__plain plugin-autotooltip_bigFluoxetine

Fluoxetine is a Selective Serotonin Reuptake Inhibitor (SSRI) used in the treatment of Major Depressive Disorderand Obsessive-Compulsive disorder etc.. It's marketed under the trade names Prozac, Sarafem, Fontex and many others.

SSRIs are believed to act by inhibiting the re-uptake of the hormone serotonin (Prozac), which, to date, has not has its 'mechanism of action' fully explained. And, unfortunately, only a small percentage of people with OCD are significantly responsive to SSRI treatment (ref.)