The powerpc port boots off a boot program in an MSDOS filesystem.  You
need to start by making an MSDOS filesystem that the OpenFirmware will
accept.

Boot OpenFirmware on the machine intended to install the system on.
it is necessary to prepare the disk that OpenBSD will be installed
on in the following way:
at the ok prompt type

        1 fat-partition hdiskX

where hdiskX is the hard drive intended for installation.  On some
machines you need to use the name 'diskX' instead of 'hdiskX'.

The boot program (/usr/mdec/ofwboot) will go into this filesystem
(/msdos/ofwboot).

Your hard disk is now prepared to have OpenBSD installed on it, and
you should proceed with the installation instructions.