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Tested on macOS 10.13 with OpenBSD 6.3

Prepare bootable USB drive with OpenBSD installer on macOS

Download the installer and verify its checksum:

$ cd /tmp
$ export URL=https://cloudflare.cdn.openbsd.org/pub/OpenBSD
$ curl -Os $URL/6.3/amd64/SHA256
$ curl -O  $URL/6.3/amd64/install63.fs
...
$ grep install63.fs SHA256|cut -f4 -d' '
df19266be16079ccd6114447f7bb13bdedb9c5cb66ecc1ea98544290fa4dc138
$ shasum -a 256 install63.fs|cut -f1 -d' '
df19266be16079ccd6114447f7bb13bdedb9c5cb66ecc1ea98544290fa4dc138
$

Plug in the USB drive. Its size should be at least 400 MB. Run diskutil list to find an identifier of the flash drive. Usually it's /dev/disk2.

Replace /dev/diskX with the identifier of the flash drive.
All data on /dev/diskX will be erased!

$ sudo diskutil unmount /dev/diskX
$ sudo dd if=install63.fs of=/dev/diskX bs=1m
...
$

Wait few minutes.

Install OpenBSD.