OpenBSD installation process for APU2 / APU3
Download the OpenBSD installer image
Choose amd64/install62.fs, file-system image (not ISO!) from here: https://ftp.eu.openbsd.org/pub/OpenBSD/6.2/amd64/ (or other mirror)
Burn the image on a USB drive
On linux you may use command similar to this one:
dd if=Downloads/install62.fs of=/dev/sdXXX bs=4M; sync;
If you are using Windows, use Rufus: https://rufus.akeo.ie/
Boot from USB
Connect the USB to your APU board and setup the Serial connection.
On Linux you can use screen or putty to setup the serial connection. For example run this as root:
# screen /dev/ttyS0 115200
If this step worked correctly you should be seeing now booting sequence.
Once you see the boot> prompt type the following two lines and press enter
stty com0 115200 set tty com0
This was the trickiest part, the rest is simple. See the full installation screendump here: