@Sam *****
There are 2 primary retirement visas, the 90-day non-O visa, and the 1-year multiple entry non-OA visa.
The biggest difference is the requirement for funds in Thailand.
With a non-O visa, you will get a 90-day visa. You can get this by showing 65,000 baht per month of retirement income to the Thai embassy, or by showing the equivalent of 800,000 baht to the Thai embassy. There is no insurance requirement, and not really many other requirements at all. Once you enter Thailand you will receive a 90-day permission stamp. You will need to act quickly once you arrive, to open a Thai bank account and transfer the 800,000 Thai baht required to get the extension into this account. The reason you need to act quickly is because the money must be in the account for 2 full calendar months already when you apply for the 1-year extension. That means you have less than 30 days to get it all taken care of to give your money time to be in the account long enough for the extension. The requirements after you receive the extension are that your bank account must remain at 800,000+ for 3 months after you receive your extension, and then can never drop below 400,000 the rest of the year. The account must be brought back up to 800,000 a full 2 calendar months before you apply for your next yearly extension, and in most offices you can apply 30 days before your stamp ends. That means you would want to get your account back up to 800,000 about 3 months before your extension ends. Most people just leave it at 800,000 so they don't have to try to keep track of the time and potentially get denied from applying for their next extension by timing it wrong.
The non-OA visa requires a lot more than a non-O visa. You will need a health certificate signed by a doctor, a criminal records check, proof of mandatory insurance, and the same proof of funds as the non-O visa. Once you get the non-OA and enter Thailand, you will receive a 1-year entry stamp, or until the end date on your insurance if less than a year. Every time you enter before the end date on the visa (not the entry stamp), you'll receive a 1-year entry stamp, or until the end date on your insurance. That means if you get another year of the required insurance, then leave and return just before your visa expires, you can get a 2nd year of permission to stay out of the single visa. If you plan to return home at least every 2-years to apply for a new OA visa, and don't mind paying for the mandatory insurance every year, then an OA might be an okay option since it doesn't require you to keep the 800,000 in a Thai bank account. But if you ever plan on staying in Thailand full time, the OA is not a good option. That's because if you try to apply for the annual extension in Thailand from a non-OA visa, you will be required to have a new year of the mandatory insurance every year. So you would have the worst of both visa types, the requirement to have the money in the Thai bank AND the required insurance.
That's why most people go for the non-O. They don't want to pay for the expensive mandatory insurance every year if they decide to stay in Thailand.