The system might be set to use a serial console. Try connecting a null
modem cable between the O2 and another machine (PC, Mac, Linux, whatever
you have handy) running a terminal emulator and see what that says. You
may find an error message about failed graphics causing it to fall back
to a serial console or maybe that it's configured for one regardless.
If you haven't already, check out both http://forums.nekochan.net/
the wiki there for more info about diagnosing and fixing SGI systems.
Those forums are much more active than this newsgroup. I have several
SGIs but I don't have any hands-on experience with an O2. So,
unfortunately, I can't offer any more advice than that.