This study is investigating whether psilocybin (an active compound in magic mushrooms) can help people with obsessive-compulsive disorder (OCD) by improving their mental flexibility and brain plasticity. Participants will receive two doses of psilocybin—a low test dose and a slightly higher therapeutic dose—four weeks apart, while receiving professional psychological support. Researchers will measure changes in cognitive abilities and brain activity to understand how psilocybin might help reduce OCD symptoms like intrusive thoughts and repetitive behaviors.