OpenGL is cross-platform. The only thing you need to worry about are the operating specific initialization, such as wgl for Win32.
SDL is also cross-platform.
gcc is cross-platform. Look into the ports mingw for Win32 and xcode for OS X.
Unfortunately most game development discussion is Windows specific. However, for Mac game development, you can look at Apple's game developer resources. Also, if you don't want to work on the Mac yourself, Red Marble Games can offer to handle the porting for you.