Mounting an iso image in OpenBSD, taken from:
http://www.openbsd.org/faq/faq14.html#MountImage
Read the manpage for vnconfig(8) for more details.

vnconfig -c /dev/svnd0c YOUR.ISO
        mount /dev/svnd0c /mnt

Unmounting the image:

umount /mnt
vnconfig -u svnd0c