• At least 18 years old

• Fluent in speaking and reading the predominantly used or recognized language of the study site

• Able to swallow pills

• Meet the criteria for OCD diagnosis

• YBOCS total score of at least 16

• Not on psychotropic medications 1 month prior to study enrollment

• Able to tolerate a treatment-free period

• Able to tolerate study procedures

• Failed at least 1 prior trial of standard first-line OCD treatment

⁃ Agree to the following lifestyle modifications: comply with requirements for fasting and refraining from certain medications prior to the Experimental Session, not enroll in any other interventional clinical trials during the duration of the study, and commit to medication dosing, therapy, and study procedures.