This paper discusses the formal description of a real time application, viz a Fault Management System (FMS) for the OSI environment, using O-Estelle. O-Estelle is an object oriented specification language that has been proposed by the authors for describing real time applications. O-Estelle is an Estelle based language with extended features. A distributed FMS is a set of Fault Managers running concurrently on several systems. It is the responsibility of the Fault Manager (FM) on each system to detect, isolate and possibly rectify the faults in the system(5). Management services are provided to the FM to perform its tasks and to communicate with FMs on other systems. We have argued that the formal description of such a distributed application requires additional features in description languages like Estelle. It will be difficult to describe the FMS using Estelle because of the non availability of appropriate features. O- Estelle description of the FMS can be used for protocol verification and semiautomatic generation of software for FMS. © 1992 IEEE.