Sources:
After installation, if you want to boot your newly installed OpenBSD, you’ll have to enter Open Firmware by pressing Command+Option+O+F at boot, and then issue the following command: boot hd:,ofwboot /bsd
Because OpenBSD is the only operating system on the machine, you’ll probably want to boot directly into it, without manually going trough Open Firmware. To do this, enter in Open Firmware again and type this:
setenv boot-device hd:,ofwboot /bsd
reset-all
If you want to run X, check if machdep.allowaperture is set to 2 in /etc/sysctl.conf. This will allow X to run accelerated.