Summary

The official documentation says:

If you have a running OpenBSD system, bsd.rd is all you need to reinstall or upgrade to a newer version.

This post shows how to use bsd.rd above, the ramdisk kernel, to start to reinstall the OS.
Network or external media is unnecessary. Besides, in installation, it is necessary to get the file sets from any of cdrom disk http nfs at "Location of sets?".

Description

Reboot the machine, enter /bsd.rd (or just bsd.rd) when boot> is shown.
Well, remember the boot process will go on automatically in a short while if you don't enter anything.

That's it !! You will see the installer start.

Conclusion

After installing OpenBSD once, bsd.rd is in your hands.
With it, you can reinstall the machine. You can also apply full disk encryption via the shell beforehand.