Brain Network Changes Accompanying and Predicting Responses to Pharmacotherapy in OCD

This study is using brain scans (fMRI) to understand how brain networks change in people with OCD (obsessive-compulsive disorder) when they are treated with the antidepressant fluoxetine (Prozac). It also includes healthy volunteers for comparison. **You may be eligible if...** - You are an adult (any gender) - For OCD group: you have a confirmed OCD diagnosis, moderate-to-severe symptoms (Y-BOCS score of 16 or more), are not currently on psychiatric medications (except occasional sleep aids), and are seeking treatment with fluoxetine - For healthy volunteers: you have no current psychiatric diagnosis and no close family history of OCD or related conditions - You are willing and able to undergo MRI scans **You may NOT be eligible if...** - You have an unstable medical or psychiatric condition requiring urgent treatment - You have had a substance use disorder in the past 6 months - You have a history of psychosis or mania - You are pregnant - You have metal in your body or severe claustrophobia (preventing MRI) - For OCD group: you previously tried fluoxetine at an adequate dose without improvement, or are allergic to SSRIs Talk to your doctor to see if this trial is right for you.