Inclusion Criteria:
1. Primary DSM-5 diagnosis of OCD
2. Y-BOCS score of 19 or greater
3. Failure of at least one trial of standard care treatment (medication and/or psychotherapy \[CBT/ERP\]) for OCD
4. English proficiency and fluency, and ability to understand the consent process and provide written informed consent
5. Willingness to sign a medical release for direct communication between research staff and external provider(s) about the participant's treatment and medical histories
6. Non-consumption of SSRIs for at least 8 weeks at the time of randomization
7. Willingness to refrain from psychiatric medications (e.g., antidepressants, first- and second-generation antipsychotics, mood stabilizers) during the study period, as well as certain other medications (e.g., anti-seizure medications, cardiovascular medications, and aldomet specifically) during the day of dosing
8. Willingness to abstain from THC-containing products for study duration. A negative urinary drug screen is also required at baseline and the day of dosing.
9. A negative urinary pregnancy screen at study entry and day of dosing if of childbearing potential, and willingness to use adequate birth control for study duration
10. Having a contact person who is willing and able to be reached by the study team in the event of an emergency/crisis, and who is able to transport the participant home at the end of the inpatient stay/dosing week
11. Willingness to commit to all study procedures and visits, including inpatient stay, assessments and self-reports, neuroimaging, and being medically cleared to be discharged and transported home at the end of the dosing week
Exclusion Criteria:
1. Personal or immediate family history of schizophrenia spectrum and other psychotic disorders, bipolar I or II disorder, or major depressive disorder with psychotic features
2. Active suicidal intent
3. Unremitted Tourette syndrome
4. Autism spectrum disorder
5. OCPD or BPD
6. Current substance use disorder (except mild alcohol use disorder)
7. Unstable neurological or medical condition(s) that may render study procedures unsafe, including poorly managed diabetes, hypertension, or cardiovascular conditions, or history of seizure(s) or chronic/severe headaches
8. Any history of head injury with loss of consciousness for more than 30 minutes
9. Any contraindications to undergoing an MRI scan, including having metal implants or metal fragments in the body
10. Any use of psychedelic substances within the prior 12 months
