I came in 2 years ago on an OA visa from USA. I submitted all the required documents while in the US, which included my Social Security letter stating my annual & monthly income. I did not have to deposit 800k and then change over, they accepted the Social Security letter as proof of the 65K requirement as our embassy does not furnish proof letters. As you will have to have health insurance from Thailand, I suggest you find them before you travel here. You can price/check their coverage from your home country., online, very easy. They will ask for the same documents as the Thai embassy, as well as additional health tests, which gives you a second check on your documentation. You will see that they are able to not cover pre-existing and other items that may be covered currently. As you are aware the 65K method also requires proof of being free of certain ailments, which you will need to get signed off from your doctor Some of the tests are not done anymore in our countries, so leave yourself enough time to get them done. The health insurance is not required for the O visa of 800K which is one of its benefits. As mentioned, you will be having your retirement 65K deposited directly into your Thai account, as proof of international funding is required to renew each year, and a letter is obtained annually from your Thai bank. Hope this helps....

I came in with using my social security letter for the monthly. You are allowed to use it for the 1st year, you do not have to wait and only use it for the second extension.

Since you are USA, if you are using Social Security retirement, you will get the documentation letter confirming your retirement amount, which shows your monthly. If you are meeting it with other pensions/etc. then they need to be shown that they come in monthly above 65,000. This is filed with you application for O-A retirement.