In order to effectively support formal representation and reuse of operation standard knowledge, a new operation standard expert system called OS expert system, has been developed based on the integration of ontology and case-based reasoning (CBR). This paper emphatically introduces operation standard ontology, case representation model, case retrieval model and case revision model. Through importing ontology theory into CBR, users can describe cases using any concept or instance of operation standard ontology, which is easier to retrieve and revise cases on the semantic level. The experimental results show that compared with the traditional CBR system, OS expert system has better recall and precision, and it increases the efficiency of establishing operation standard.