I have gone with using SDL for porting the game to OSX. It is easy to use, and cross platform, so if need be the game could be ported very easily to other platforms.
The file input and output will not be cross platform as I am using Carbon functions for these things. I also now support BIG_ENDIAN and LITTLE_ENDIAN platforms, so that it can run on Intel and PPC macs.
For graphics I will be using OpenGL, but I haven't decided what to use for sound yet, possibly OpenAL, but that restricts which versions of OSX can run the game (OpenAL only started being bundled with OSX 10.4). So far the game should run in 10.2 and onwards (which meets my aim).
In my spare time I have decided to learn network programming in C. I had previously created basic MUDs in perl, but never in C. I have glanced at a few tutorials in the last couple of days, and plan to read Ron's MUD Game Programming book, but so far I have managed to get a little multiplayer online game up and running, so far no client (other than telnet) but maybe in the future I will get an Ogre client that will connect to the server so its possible to move around in 3D.