I can't know for sure what's wrong precisely because when I tried more than 1 year ago OpenBSD on my iMac G5 it refuses to boot. Action retro did a couple videos regarding the iMac G3 and OpenBSD.
You know that are some different revisions of this machine? So maybe that's one reason because one config file not necessarily should work if the hardware it's different.
Since he didn't did a step by step detailing and/or having a writing guide, I must assume that everything important/relevant it's recorded. So the packages/drivers required by Xorg are installed somehow?
The problem with using files of configurations from somewhere, it's that you don't know what specific version of OpenBSD was this targeting, and xorg version can be different, and so maybe some parameters are incompatible. So maybe that's your problem.
The
first video the installation begins at 07:50 and the xorg file it's hosted in his domain (
http://frogfind.com/files/bsd/xorg.conf)
The
second video he uses a different model, the installation begins at 08:40. Since it's a different hardware the config it's different (
https://gist.githubusercontent.com/...6844bf1320193e3158258657dc43954a0c2/xorg.conf) and that's what you are using.
So that's here where my knowledge ends and I can help you troubleshooting.
Good luck
