Currently, the only people I trust for fdisk'ing are the guys from GRUB.
You can effectively partition a hard drive via GRUB.
As an other method, *always* use the OS implementation of fdisk to write it's *own* partition.
On any OS, the fdsik implementation will assume it hols the "real truth" and f*ck with master or volume boot records. (Fwiw, there is no such thing as a partition boot record, there are volume boot recors: a volume is not necessarely a partition).
Gparted (and the commercial versions) are brain dammaged. They speak windows and linux. That is all.
So, getting a f*cked-up "partition" to be able to boot is just the matter of having either a MBR or VBR at the right place.
On OpenBSD the needed files are under /usr/mdec
Dual booting Windows and OpenBSD can be done with the Windows NTloader
http://www.winimage.com/bootpart.htm
will give you directions.
Anyway, I believe GRUB really is worth learning. There are versions to boot and install from floppy or CDROM. And once the tool is there and you know how to use it, you'll always be able to circumvent any OS f*ck-ups.