@Ian ********
They are vastly different. The non-O is a 90 day single entry visa, the best for long-term stay in Thailand because the requirements are very low. Just need to show 800,000 baht or equivalent in your home country currency basically. Then you can apply for annual extensions in Thailand showing 800,000 baht in a Thai bank and a little paperwork.
The non-OA is a 1-year multiple entry visa. It has very high requirements including a medical check, criminal background check, mandatory insurance, and others. The non-OA only makes sense if you really don't want to keep 800,000 baht in a Thai bank but instead keep it in your bank at home. You'll have to pay for the mandatory insurance forever, from a Thai company. And you'll have to go home at least once every 2 years to apply for a new one from your embassy. If you want to try to get an extension in Thailand, you will have the exact same requirements as the non-O extension, except you'll also need the mandatory insurance, making it the worst of both worlds in that regard.