Studies show that hormones affect the brain's fear extinction network, which is relevant for therapy involving exposure and response prevention (EX/RP), a first-line treatment for obsessive compulsive disorder (OCD). This study will examine the effect of delivering EX/RP to women during different phases in their menstrual cycle to determine the effects of hormones on the fear extinction network and on their OCD symptoms.

This study involves neuroimaging and a brief EX/RP protocol. The investigators will recruit both male and female participants. All participants will complete neuroimaging, hormonal assays and symptom ratings before and after a brief course of EX/RP. Female participants will be randomized to complete brief EX/RP during distinct phases of their menstrual cycles. This design will allow the investigators to study the effects of hormonal variation during the menstrual cycle and sex on the fear extinction network and on EX/RP outcome. The long-term goal is to optimize EX/RP for individuals suffering from OCD. Men will also be recruited for this study to further explore biological sex differences. The results will elucidate treatment mechanisms and could lead to personalized treatment recommendations for women with OCD.

Inclusion Criteria:

1. Diagnostic and Statistical Manual of Mental Disorders (DSM-5) Diagnosis of OCD;
2. Male and female subjects aged between 18- 45;
3. Women must be menstruating and regularly cycling
4. Ability to tolerate a treatment-free period;
5. No psychotropic medication in the past 12 weeks
6. At entry, at least moderate severity OCD
7. Willingness and ability to give written informed consent after full explanation of study procedures.

Exclusion Criteria:

1. Use of birth control (oral contraception or IUD) that affects the menstrual cycle, or menopause.
2. Pregnancy. Women of childbearing potential will be required to sign a statement indicating their intention to avoid pregnancy during the study.
3. Neurologic or medical condition that would prevent safe participation in the full study protocol.
4. Any contradiction to magnetic resonance imaging (e.g., metallic implants or devices).
5. Comorbid psychiatric conditions that significantly elevate the risks associated with study participation or confound results.
6. Patients with prominent suicidal ideation or with a recent suicide attempt.
7. Current psychotherapy

Study hypotheses will be masked to participant, therapist, and outcome assessor. Assignment of women participants to groups will be masked to therapist, investigator, and outcome assessor.