Hi Tenoch, I'm the proud owner of several early P1-class systems like that, PCI interrupt routing is always.. sketchy.
Some other options you might wish to try: * Look in the BIOS for a setting similar to "Plug & Play OS", set that option to "No".. this will force the BIOS to setup some hardware prior to passing control over to OpenBSD. * Try a 4.5-CURRENT snapshot, some related changes were made that might help things. Good luck.. -Brynet.

