Okay, OpenFirmware, to the best of my knowledge, is the firmware of every New World PowerPC Mac. This may sound stupid, but it's very important to understand that this is firmware, not software. Firmware interfaces with the hardware, if there isn't the ability for the hardware to interface with the firmware, it will be like nothing is even there.
Now, on PC's the firmware is BIOS or EFI, to my knowledge, EFI is usually 100% compatible with BIOS. Cards interface with BIOS and EFI in a way OpenFirmware isn't compatible with.
This is why PowerPC mac users need different cards. If a BIOS/EFI compatible card (All modern ones except ones made before 2006 specifically for macs) is inserted into a OpenFirmware computer (Apple Macs made before Intel switch and some IBM POWER computers) it won't recognize it. Since it's firmware, and not software that isn't recognizing it, it doesn't matter what operating system your using (Linux, Mac OS X, BSD) even if they have the correct drivers to support the graphics card.
I hope this made sense.
BTW: Lil Chibil was saying "if it were osx" not "if it were compatible with osx"