Should I apply for a Non-Immigrant O or OA visa for a 9-month stay in Thailand?

If you get an OA you can use money in you home country bank accounts. Once you get the OA you just arrive and there is nothing else to do. If you come on a non-O then then while you are here you will have to open a Thai bank account (if you don’t already have one), transfer the money, and then go to Immigration to apply for your year extension. Question: do you want to be occupied doing that after you arrive?

a Non-O visa (usually sold only as a single entry) is good for a 90 day entry stamp when you come in and then you'd need to meet the proof of funds (800K baht in a thai bank account in your name only for 2 months before you applied for the extension) to get a year extension from it. There is no extension shorter than a year on a Non-O for retirement.

OR

You get the year-long, multi-entry Non-Immigrant Type O-A (Long Stay) Visa from the thai consulate in your country <- the one that requires the police back ground check, the medical certificate, the proof of funds (using your bank there) the proof of both 40K/400k out/in patient health insurance and 100K USD in covid insurance and when you come in you'll get stamped in for a whole year (or as long as your insurance is good for,).