This study tests whether psilocybin (the active compound in certain mushrooms) can help people with obsessive-compulsive disorder who haven’t responded to standard treatments. In a carefully controlled trial, participants receive either a single dose of psilocybin or a placebo while receiving psychological support, with their brain activity and symptoms monitored. The research aims to determine if this novel treatment is safe and whether it could work faster than existing medications for this difficult-to-treat condition.