Most MACHINE machines don't generally need any specific preparation.

On series 400, you will wish to configure the prom as ``HP-UX compatible''
before installing OpenBSD. To do so, at the prom prompt, enter ``CF'',
then ``2'' (Boot Mode Selection), then ``2'' (HP-UX Compatible).  Choose
to make this setting Permanent with ``P'', then validate your changes with
``E''.  Your machine will restart in HP-UX mode.

If you plan to use a serial console, you might have to flip a switch
on series 300, or configure the RS-232 interface as ``remote'' from the
Configuration menu, on series 400.  Please refer to the OpenBSD/MACHINE
FAQ at http://www.openbsd.org/hp300/hp300faq.html for more information
about this process.