OS/2 API supported by OS/2 Personality server. It is consist of set of dynamic-linked libraries whith APIENTRY calling convertion. Applications can link required functions set to prefom actions. API consist of two generic parts:
Classic API is set of functions which can be directly used. It is classic approach used by many OSes.
Object-oriented API is SOM-based API. System Object Model is a core of this API. It can be used by any language (not only by object-oriented languages), but requires additional layer and build tools.