Okay - hopefully you have not put too much time into your OpenBSD install.
When I installed OpenBSD, with the fdisk portion, I left my OpenBSD partition marked active; this is under the pretense that you do not have a silly BIOS, and that the BIOS looks at the
MBR of the disk and not the PBR of the first partition.
Check out
this section of the OpenBSD FAQ.