Can I invest in or own a company while on an OA retirement visa in Thailand?

You are absolutely allowed to own or invest on that type of visa. As long as you don’t actually work for the business it’s not an issue. Way back there used to be issues with “owners” and what’s determined as “working” as in if you signed a cheque for that company you could be classified as working for it, but I think that’s way in the past and Thai authorities have moved on from that.