Install on Apple Silicon:

  These machines do not come with UEFI firmware by default.  In order
  to install OpenBSD on these machine you need to run the Asahi Linux
  installer first in macOS or the macOS recovery environment.
  Instructions on how to download and run the Asahi Linux installer
  can be found at https://asahilinux.org/.  Run it in macOS or the
  macOS recovery environment.

  Note that while running the suggested:
      curl https://alx.sh | sh
  is probably safe, it is teaching a bad habit.  It is better to
  download the installer:
    curl -o alx.sh https://alx.sh
  and inspect the alx.sh script before running it:
    sh alx.sh

  You will have to create some free space for your OpenBSD install by
  choosing the
    "Resize an existing partition to make space for a new OS"
  option first.  Once that is done, pick the
    "Install an OS into free space"
  option, choose
    "UEFI environment only (m1n1 + U-Boot + ESP)"
  and follow the steps presented to you.  

  Now you can copy the miniroot or install image
  ("miniroot73.img" or "install73.img") to a USB
  drive, plug it into one of the type-C ports on the machine and reset
  the machine to boot into the OpenBSD installer.