Investigators will recruit 36 individuals on MAT for OUD for a double-blind, placebo-controlled design to determine whether PEX010 (25-mg/d) shows preliminary efficacy on neural correlates of neurocognition and on clinical outcomes. Participants will be randomized to either (single dose) 25-mg (PEX010-25 group) or 1-mg (PPEX010-1 group) PEX010 in a 2:1 ratio. Brain and behavioral testing sessions will precede Psilocybin (PSI) dosing day by 24-48 hours and will follow PSI dosing by 1 week. After an initial 6 phases, participants will come into the lab to submit a urine screen 2x/week and to complete a short survey in order to collect data on drug use, MAT adherence, and mental health symptoms. The investigators hypothesize the PEX010-25 (vs. PEX010-1) group will have better clinical outcomes (e.g., lower average percent positive urine drug screens, more late relapses, higher MAT adherence). There are research follow ups every three months out to one year post dose.