tested on macos 15 and openbsd 7.8

prepare bootable usb drive with openbsd installer on macos

download and verify the installer:

$ cd /tmp
$ x=https://cdn.openbsd.org/pub/OpenBSD/7.8/amd64
$ curl -Os $x/SHA256
$ curl -Os $x/install78.img
$ shasum -c SHA256 --ignore-missing
install78.img: OK
$

plug in an empty usb flash drive and find its identifier:

$ sudo diskutil list | grep external | cut -f1 -w
/dev/disk4
$

replace /dev/diskX with the identifier of the drive.

warning! all data on /dev/diskX will be erased.

$ sudo diskutil unmountDisk /dev/diskX
Unmount of all volumes on disk4 was successful
$ sudo dd if=install78.img of=/dev/diskX bs=1m
800+1 records in
800+1 records out
839352320 bytes transferred in 197.022927 secs (4260176 bytes/sec)
$