An identical twin with severe OCD reported lower symptom severity, better emotional regulation, and improved well-being after taking 1–5 mg psilocybin every third day, while the unaffected twin took none. Despite the perceived improvements, cognitive flexibility deficits still remained compared with the non-OCD twin during set-shifting tasks, suggesting the microdosing routine did not normalize underlying cognitive rigidity. This is a single observational twin case, not clinical proof, but it adds rare real-world data to the growing conversation around psilocybin and OCD.