The paper investigates the applicability of the object-oriented technique to large and complex systems as exemplified by the operating system BS2000 which has been under constant development for a number of years. The proposed system architecture ensures the harmonious coexistence of procedural and object-oriented parts of the system. New domains, which are implemented using the object-oriented paradigm, can be smoothly embedded in the existing system. The parallel usage of different implementation languages is rendered economically viable. In our framework some representative parts of the system were redesigned and implemented in a prototype. The extensibility of the design was checked by including further parts into this scheme. The results are encouraging, so that the object-oriented technique will be used in the further development process. The proposed technique can also be applied to systems with a different structure, even a monolithic one.>