For what it's worth, probably not much, openbsd still has raw devices /dev/rsd1a vs /dev/sd1a(the block device) while it looks like linux made them all block devices (unless opened a special way O_DIRECT) and freebsd(as best I can tell) made all devices raw. https://docs-archive.freebsd.org/doc/11.2-RELEASE/usr/local/...

So, on openbsd, make sure you use the raw device when writing images with dd, and in my experience you want to use dd because the block size on cat is too small. dd bs=64k if=image.file of=/dev/rsd2c (c on obsd is entire disk)