There are a number of ways of obtaining OSLib, and the OSLibSupport library, depending upon your requirements.
First, the OSLib Library in binary form, complete with C and assembler headers, is available for download as a zipped archive, for download. This is all you need to link with your own applications to provide access to all the RISC OS SWI calls from assembler or C.
Secondly, the OSLibSupport library, again in binary form, complete with C, is available for download as a zipped archive, for download. This is a C library of support functions, designed to make programming the RISC OS WIMP and Toolbox modules as easy as possible.
Third, a StrongHelp manual covering the entire API from C is available for each version.
Finally, the complete source for the project is now available via SVN from SourceForge. This includes all source files, together with the tools necessary to build the library. In addition, Makefiles are provided to build the tools and the library. From OSLib 7.00 onwards only GCCSDK is supported as build tool via its cross-compiler.