Although referenced in the documentation:
cdrom42.fs The i386 boot and installation 2.88MB
floppy image that contains almost all OpenBSD
drivers; see below.
the cdrom42.fs image is no longer included in OpenBSD. If you want to make your own custom OpenBSD cd without having to manipulate the 200MB iso, you can simply use the older cdrom41.fs:
See http://unixsadm.blogspot.com/2007/08/master-openbsd-iso-image.html for details, but use the 4.2 install sets.
mkisofs -vrTJ -V "OpenBSD42" -b 4.1/i386/cdrom41.fs -c boot.catalog -o OpenBSD.iso ~/OpenBSD
Yep, you can use cdrom41.fs and it will just work.