Brain Network Changes Accompanying and Predicting Responses to Pharmacotherapy in OCD

Inclusion Criteria: * Inclusion criteria for all groups will include: (i) Adults, Men and women will be included, in a 1:1 ratio; and efforts will be made to recruit racial and ethnic subgroups. Groups will be further matched on age, handedness; and later, in analysis, on educational status and smoking history. (ii) signed informed consent, approved by the Yale Human Investigations Committee; and (iii) ability and willingness to participate in all study procedures, including clinical assessments and fMRI scans. * Additional inclusion criteria: * OCD Participants: (i) a DSM diagnosis of OCD, established as detailed above; (ii) a baseline Y-BOCS of ≥16; (iii) no current psychoactive medication, with the exception of the occasional use of a PRN medication for sleep; (iv) treatment seeking, and clinically appropriate for fluoxetine pharmacotherapy; (v) for women of childbearing potential, not pregnant or intending to become pregnant, and willing to use reliable birth control over the course of the study, either prescribed contraceptive (oral contraceptive, injectable, or implant) or one barrier method (e.g. diaphragm with spermicide, intrauterine device, cervical cap). Women of childbearing potential must have a negative serum pregnancy test at screening and negative urine pregnancy tests at the study baseline and on each scan day appointment prior to the MRI scan. * Healthy Control Participants: (i) no current DSM diagnosis (by clinician interview confirmed by SCID or MINI); (ii) no documented or clinically suspected family history in a first- or second- degree relative of OCD, Tourette syndrome, hoarding disorder, body dysmorphic disorder, or a compulsive grooming disorder. Exclusion Criteria: * Exclusion criteria for all groups will include: (i) any unstable medical, psychiatric, or neurological condition (including active or otherwise problematic suicidality) that may necessitate urgent treatment; (ii) any substance use disorder within the past 6 months; (iii) use of psychotropic medications within the past 8 weeks, with the exception of occasional use of a sleep aid or analgesic; (iv) use of an as-needed sleep aid or prescription analgesic within 3 days of the scan; (v) any history of a primary psychotic disorder or of mania; (vi) any evidence of substance use on urine toxicology testing; (vii) any major neurological disease or history of major head trauma, including concussion with extended loss of consciousness; (viii) pregnancy; (ix) any metal in the body or other contraindication to MRI scanning; (x) severe claustrophobia, back pain, or other condition that may make an extended MR scan difficult or lead to excessive movement during the scan. * Additional exclusion criteria: * OCD Participants: (i) past allergy or adverse reaction to fluoxetine or another SSRI, or other clinical contraindication to fluoxetine pharmacotherapy; (ii) documented nonresponse to a past trial of fluoxetine of appropriate dose (≥40 mg/dy) and duration (≥12 weeks). (iii) Patients taking Coumadin or Monoamine oxidase inhibitors (MAOIs) will be excluded from the study.