What financial proof is required for an O-A visa application in Thailand for US citizens receiving Social Security?

You can not use any document from SSA to prove your financial requirement. As a US passport holder, the only documents immigration accepts are your official bank statements, and not saved each month, but a 12 month summary. As a retired US citizen I have my SS direct deposited into my Bangkok Bank account in Thailand, and it no longer goes through the New York branch doing this, but is sent by the SSA directly into my Thai branch account. Do not get the type OA retirement visa. Wait until you get here, get your Thai bank account, transfer at least bt800,000 into it from an overseas source, for a few months, two at least, then apply for the type O retirement visa extension. After the first year you can use the monthly deposit method, but the first year your only option is having at least bt800,000 in the bank. This even applies if you use the type OA visa option from the US. You must show at least bt800,000 in your bank account. The main reason you want the type O retirement visa extension rather than the OA is because of the required health insurance. Not that you shouldn't have health insurance, you should, but because if there is any problems with your health insurance, like they cancel you as sometimes does happen, the length of your entry stamp, up to one year, is based on your insurance validity. You want health insurance, but you do not want it's validity tied to the length of time you can stay, and the effect of losing your insurance could have on your legal entry stamp. It is very easy to do after the first year, but to get through that first year you must have a minimum of bt800,000 in your bank account

You get an OA visa from the Thai embassy in your home country. You need to check with them on what they require for an OA visa.

If you're talking about getting an extension in Thailand, then you cannot use proof of income for an extension if you're American. The ONLY thing the immigration office will accept is certification of income from the US embassy and they stopped providing that about 5 years ago.

The only thing you can do is show proof of transfer from overseas to your Thai bank account in the amount of at least 65,000₿ every month for the previous 12 months before you apply.

You also cannot use this method to convert to a non-O or for your first 1 year extension.