I found this with no less than 1 search on Google
Bypassing Open Firmware Security
If you forget the Open Firmware password for a computer, or if someone else sets one before you do, you can reset the Open Firmware settings to their defaults (no password and the none security mode). If other Open Firmware settings have been changed, this might reset them as well. As I said, it's a cumbersome process, so you'll want to avoid having to do it.
First, open the computer and either remove or install RAM. What you need to do is change the amount of RAM that's installed in the computer, so simply moving modules around won't do the trick. Reboot the computer with the changed amount of RAM and zap the PRAM. (Changing the installed RAM allows you to use the command-option-P-R key combination to zap the PRAM, regardless of the Open Firmware security mode, which removes the password.) Then boot into the Open Firmware prompt and use the set-defaults command. This should reset all Open Firmware configurations to the default settings. Use the reset-all command to reboot with the new settings, after which you can set a new password and security mode (either directly through the Open Firmware prompt or using another tool). Restore the original amount of RAM in the computer.