Main objectives of an Onboard Maintenance System are to support aircraft troubleshooting in order to meet customers operational needs, to minimize direct maintenance cost and direct operating cost, and to maximize operational reliability. How to design Onboard Maintenance System is very important. This paper introduces OMS philosophy, system safety, fault processing, system interfaces, and maintenance process using an OMS, human machine interface, in order to help readers understand OMS better, broaden design ideas for successful design OMS.