The OSL (Operating System Layer) allows cross platform access to common operating system features but that are accessed differently depending on the platform. The OSL consists of a series of C++ classes, backed by a C-based API for maximum portability. This was necessary when Star Division started because many of the things needed, such as mutexes, condition variables, threading, time functions and a variety of other operating system specific mechanisms hadn't been produced by libraries such as the STL, and certainly what there was couldn't be used as a cross platform solution.
As the C++ Standard Template Library has matured, there have also been other libraries that work in a cross platform manner (such as Boost) and so large parts of the OSL (or all of it) is largely better handled by these libraries. The following will discuss the functionality implemented in the OSL, with a comparison against more modern, cross platform C++ libraries.