When submitting a run, would an OBS video file (.mp4) work, or is it too much of a hassle for mods?

You cannot submit mp4 files as the moderator has no way to view it.

That OBS video file, upload that to YouTube or another website that'll permanently store your video. That works.

@LunchBox1211 just upload that mp4 to youtube. You can make an account in seconds and upload for free. You can upload it unlisted so it can only be viewed with a link, but considering everyone needs to see your run for it to be valid, I'm not sure what the point would be.