OpenGL for UT is not "supported" by Epic. I have to manually select it also for my GeForce card by going to Preferences and selecting Change Renderer, the selecting Show All (Its' been a while since I did this so I don't remember the exact names for the steps), however, UT does not crash when I restart the game.
As for as UT selecting OpenGL as the renderer for you graphics card, it won't do it as far as I know. Remember, UT was originally designed for Glide and the D3D rendering was crappy. The OpenGL rendering was total crap. Due to the large number of video cards (GeForces and Radeons)not supporting Glide, D3D support was greatly improved, but OpenGL was not. It wasn't until the programer at Loki worked on the OpenGL dll file that OpenGL rendering of UT became a viable option.
MAC