I had a similar problem with not being able to get into OSX, (or OS9 for that matter - my PBook refused to boot!!) Although I could bring up the disk selector using the option key, it wouldn't find the OSX disk.
Eventually, I found out that you can change the startup disk from the OSX install CD/DVD. If you boot into that (hold down C?), then go to the Installer menu, you can change the startup disk from there.
Hope this helps!