It may not seem to be a big deal. But correct terminology does matter, you would enter as visa exempt. Not visa on arrival. As others have mentioned, it would be best to get a Non O before you arrive, then you can get extended to a full 12 months in Thailand.
As for a return ticket on a visa exempt, your airline could ask as it is a requirement. You could buy a throw away ticket from Thailand to Malaysia, Laos or Cambodia within 60 days, whatever is cheapest. There are also dummy ticket companies where you can purchase a fake ticket for a low charge.
Thailand has mentioned plans for an ETA but nothing tangible has been done. For now, just arrive with your passport and proof of leaving Thailand within 60 days and maybe a hotel booking and have a great time.