In the case of OS9 it will install on top of it, leaving OS9. You will have a "System Folder" and "System" at the top level of the drive.
What machine is this? To use Mac OS X 10.4 Tiger, your Macintosh needs:
* A PowerPC G3, G4, or G5 processor
* Built-in FireWire
* At least 256 MB of RAM
* DVD drive (DVD-ROM), Combo (CD-RW/DVD-ROM) or SuperDrive (DVD-R) for installation
* At least 3 GB of free disk space; 4 GB if you install the XCode 2 Developer Tools
It would need to be a slot-loading DV/SE or DV/SE Summer model with 400 MHz processor or faster in order to run Tiger. This Firmware Update should be installed: iMac Firmware Update 4.1.9. You must do that while in OS 9. The link does not appear to go anywhere any more suggesting that all stuff that old is archived or no longer available.