| Ocfentanil | opioid | 39th ECDD (2017) - Critical review | 
      
                                                                      | OPIUM (note 3 page 6) | opioid | None 
 | 
      
                                                                      | Oripavine | opioid | 33rd ECDD (2003) - Pre-review, 34th ECDD (2006) - Critical review | 
      
                                                                      | Ortho-fluorofentanyl | opioid | 41st ECDD (2018) - Critical review | 
      
                                                                      | OXAZEPAM | benzodiazepine | None 
 | 
      
                                                                      | OXAZOLAM | benzodiazepine | None 
 | 
      
                                                                      | Oxpheneridine | other | 7th ECDD (1957) - Critical review , 12th ECDD (1962) - Critical review | 
      
                                                                      | OXYCODONE | opioid | None 
 | 
      
                                                                      | Oxymorphone | opioid | 9th ECDD (1959) - Critical review |