You may qualify for this study if you meet the following criteria.

Inclusion Criteria:

• 18-65 years old
• Diagnosed with or experiencing symptoms of OCD
• Have tried at least one OCD treatment (e.g. SRI or ExRP psychotherapy), or refused these treatments for individual reasons (e.g. concerned about side effects)
• Fluent in English
• Able to provide informed consent
• Able to tolerate study procedures, including nitrous oxide

Exclusion Criteria:

• Taking psychotropic medication for the past 6 weeks
• Started OCD-targeted Exposure and Response Prevention (ExRP) psychotherapy within 2 months
• Current psychosis or bipolar disorder
• Currently experiencing suicidal behaviors
• Current moderate or severe substance use disorders
• Allergy or hypersensitivity to nitrous oxide
• Medical illness that could interfere with study participation or use of nitrous oxide, including but not limited to:
  • megaloblastic anemia, myelopathy, chronic disturbances of folate metabolism, middle ear occlusion, elevated intracranial pressure, and significant pulmonary disease (e.g. reactive airway disease, chronic obstructive pulmonary disease [COPD], restrictive lung disease)
• Use of medications that might increase risk of participation
• Pregnancy or nursing
• Positive urine screen for illicit drugs

Participation may involve up to 13 study visits. These include:

• Screening
• Baseline: questionnaires & assessments, blood pressure & heart rate measurements, drug & pregnancy tests, computer tasks to test your thinking
• Administration: inhalation of the study treatment for one hour (randomly assigned nitrous oxide gas or placebo air, with 2:1 ratio of nitrous oxide to placebo), EEG (brain-wave) recording, a blood draw and physical and psychiatric exams
• Follow-up assessments