What are the eligibility requirements and processes for obtaining long-stay visas in Thailand as a US citizen?

I left the UK 19 years ago yet my pensions combined which includes a frozen OAP is taxed every month they even tax you when you die : (

Get a Non O before you come and simply renew it annually.

Thank me later.

I think it's safe to assume you are not actually eligible for the O-X visa. There's a reason no one except Australians use this visa. The consulate in Australia is the only one in the world that lets you apply with money in your home bank account. For every other Thai embassy/consulate, you must show proof of 3 million baht in a Thai bank account. Do you currently have 3 million baht in your Thai bank account? If not, you are not eligible.

Get the 90 day non-O visa like most people. That is what you can extend annually in Thailand. If you get a non-OA, the agents aren't going to want much to do with you since that one has mandatory insurance every year and you're also going to have to do a lot of paperwork in the US to apply for it.

The requirements for the non-O are trivial to obtain from the embassy, and this is the type that agents normally offer for visa/extension in Thailand.