I don't believe this is possible. The FreeBSD compat layer in OpenBSD is for userland binary compatibility and doesn't expend to kernel objects. There are a lot of differences between the FreeBSD and OpenBSD kernels, so I doubt even if you managed to get it to load the driver that it would be able to function correctly.
I doubt nvidia would put the time and effort into supporting OpenBSD, I can imagine the install base for desktop users is small even compared to FreeBSD desktop users. Plus there is a lot of history with the OpenBSD dev's dislike for binary blob drivers.
Your best shot is probably using the nv driver
that comes with your xserver, it may not be fully accellerated but it'll get you an X desktop running.