If you check this page:
you will notice that 10.2 for OSX is for Intel based Macs, not PPC.
From what I remember there was a 10.1 version for PPC available - forget it. You cannot use your old G3 for any oracle database installation. Not even in Virtual PC for mac due to the 512 MB Ram limit. Even running 10.4 on a G3 is a hack. The computer is simply too old, 10 years plus?