The interface is the same as the VGA interface for Svgalib. The extensive man pages for those functions are in oskit/video/svgalib/doc, handily formatted for you. Svgalib programs that use only the VGA interface should work with little change: you should only have to add code to do OSKit initialization.
For an example of how to convert an svgalib program, you can look at examples/x86/svgalib.c, which is one of the standard svgalib example programs converted to run on the oskit.