Before you can just replace Debian with OS X, you should make sure that the "old iMac" has the specs (preferably at least 128MB RAM for panther, and twice that for tiger). Some of the really old iMacs might have less ram (they had minimum of 64MB). Even on a G3 with 128MB it can take an os x installer cd a long time to come up.
On some old models, there needs to be a firmware update to run os x. On these models, you have to install classic first, and use classic to download and install the firmware update... then try to install os x.
The existing debian install would not interfere with an os x install, so the best place for further details would be at