when you installed OS X, did you install classic as well? (go to apple menu > system preferences > startup disk and see if there's a version of OS 9 that you can start up from.
If so, choose that volume, and you'r good to go. You will still have OS X on the machine, but without install disks I'm not sure you can do anything about that short of manually erasing OS X files, which I wouldn't recommend.
If you can get your hands on an OS 9 install disk, you can go from 9.0 to 9.2 via updates at http://www.apple.com/support