It's possible to mount ISO or IMG files under OpenBSD. However, before this is possible, you must configure a vnd device first. A vnd is a VNode disk driver and provides a disk like interface to a file. This can be used for all sorts of images, like CD and DVD images but also for builing a miniroot or older floppy disk images.
To mount a cd image called image.iso, located in /tmp/, use the following commands:
vnconfig vnd0 /tmp/image.iso
mount -t cd9660 /dev/vnd0c /mnt
The ‘-t’ option specifies the type of image. In this case cd9660.
To unmount the image and unconfigure the vnd device, do:
vnconfig -u vnd0