OPD treats and diagnoses any general disease or health condition. Emergency, on the other hand, is for specific conditions like accidents. The location, treatment, fees as well as the function of both departments differs.
Further, OPD is for non-emergency services like consultation, diagnosis and treatment of patients on an outpatient basis. Emergency is for immediate medical attention to the patients having life threatening conditions or such serious injuries that need urgent medical intervention. The emergency departments are open 24/7 and have medical teams specially trained to handle emergencies.