Are you sure the disks are specific to your Mac model? The disk should say on it that it’s for your specific computer, Apple makes disks only for specific models. Particularly because you’ve got a computer with a PowerPC processor, this makes a big difference (it makes a big difference anyway, but still). You can try booting to the disk by restarting the computer and holding C once the computer turns on, until you hear a chime, then select the OS disc from the startup options. Only try this, though, if you’re sure you’ve got a legit OS disk that you trust. What are you trying to do with the OS disk, if I may ask?