I very much doubt that just marking a partition as OpenBSD is the end of the architecture specific stuff. Somehow the firmware in your computer has to actually boot the OS. Unless things are setup in a way which is acceptable to the firmware with regards to booting (be it mandatory filesystems, directory structure and file formats, master boot records, partition table entries and corresponding flags, GUIDs, bootsectors, etc.), nothing much is going to happen when actually trying to boot it.