What platforms does the OpenSceneGraph support?
Microsoft Windows (2000, XP, Vista), GNU/Linux and MacOS/X are currently the best supported ones. Your mileage may vary on Solaris, IRIX, ...
What renderers (OpenGL, DirectX, ...) does the OpenSceneGraph support?
OpenSceneGraph only supports rendering through OpenGL. In fact, a lot of the classes in the library are merely object-oriented versions of OpenGL concepts.