VMWare:
When a VMWare guest, the
vic(4) and
vmt(4) drivers provide kernel interoperability with the host OS.
There is no longer a VMWare port/package for using OpenBSD as the host, after OpenBSD 4.4, due to conflicts between 3rd party kernel modules and these new drivers.
QEMU:
This is the most popular hardware emulator. There are others, in
ports/emulators
The kqemu accelerator is available for OpenBSD/i386, but functions only with uniprocessors.