I’m afraid I do not understand your request, however GL_ARB_occlusion_query is part of OpenGL 1.5 specification, and a G5 w ATI 9800 is able to manage this extension.
OpenGL uses layered extensions to expose functionality, so you can not figure out what a given video card can do just by looking at GL_VERSION. See Understanding and Detecting OpenGL Functionality for an overview.
This GL Info table may also be useful to see what features are available on each video card on Mac OS X.