One of the objectives of the CTRON interface specifications is to promote software-reutilization for real-time applications by standardizing operating system interfaces. Although this approach does improve reusability by improving portability, means for gaining better composability has to be taken into consideration to achieve more advanced reutilization. Object Oriented Programming (OOP) which supports encapsulation, polymorphism and inheritance is an appropriate programming paradigm for improving composability of software-modules. This paper describes how we apply OOP to systems based on the CTRON specifications and mainly describes the C++ class libraries we have been designing. This paper also describes how we evaluate the performance, reusability and portability of programs that use the base class libraries.