assuming you have a apu2|apu3|apu4 from pcengines or a virtual machine running on KVM/Qemu. And you don’t have vga/dvi/hdmi whatever kind of video output. of course, you can install and run OpenBSD (or Linux) on this boxes.
if you wanna install from an USB Stick, set the correct Port and Speed before booting.
boot> stty com0 115200 boot> set tty com0
or put these settings ins in the boot.conf on your tftp server.
echo <<'EOF'> /tftpboot/etc/boot.conf stty com0 115200 set tty com0 EOF
if you have your system installed, the /etc/ttys should look like this:
root@apu ~# fgrep tty00 /etc/ttys tty00 "/usr/libexec/getty std.9600" vt220 on secure
so, it’s all ready to get a login prompt on your console.
but if you connect to your apu through ssh and you wan’t to use the Serial Port for connecting to other’s boxes Serial Port (Switches ? install another APU ? whatever ?) you still can do that, but you need to update your /etc/ttys file and reboot afterwards
root@apu ~# fgrep tty00 /etc/ttys tty00 "/usr/libexec/getty std.9600" unknown off