Hi, os2.h is available a couple of ways. libc-devel will install a version that GCC will default to using. The toolkit has the official one, most of it can be installed with YUM/ANPM, os2tk45-headers for example will install it, with other packages starting with os2tk45 installing the rest of the toolkit including the official documentation.
For porting Posix software, use GCC. Porting a soundcard driver is much different then porting Posix stuff, as you need a 16/32 bit compiler for part of it with OpenWatcom being preferred. Not the fork on Github. It also has an os2.h