As described in https://www.openbsd.org/faq/faq13.html
I enable audio recording with sysctl, create the monitor subdevice, and
restart sndiod.
# sysctl kern.audio.record=1
# echo kern.audio.record=1 >> /etc/sysctl.conf
# rcctl set sndiod flags -s default -m play,mon -s mon
# rcctl restart sndiod
but I'm confused on the next step - how do I configure obs to use the
snd/mon device? When I open Settings/Audio all "Global Audio Devices"
are empty. I have built-in azalia, one uaudio sound card, and uaudio
headphones if it matters.