For Brits there is a document similar to the Income Affidavit - however the British Embassy in Bkk requires proof of the monthly income from whatever sources.
All in all I hope that some benefitted from the discussion. I ask my questions of the original poster (OP) because from experience over the years and just a few days ago here on this Group I felt strongly the full information about the O-A visa was not known. And felt the OP and others would benefit from posting the info. In dozens of instances I have seen the O-A Visa buyer has no way to even get a hint about the full nature of this Visa - because there is no indication of it on the Consular Services website... So what is a person to do? I enjoy educating these Visa buyers as it is like a free gift to let them to let them know of the full benefits, features and options - sort of like an early Christmas present.
One fellow in this thread said he had to get an Extension of Stay for the Second Year. Obviously he didn't know and it would seem the I/O didn't know either.
I don't find it uncommon. Over the last three years on various discussion boards I find that most do not know and are usually pleasantly surprised to find out the details. The O-A visa is not sufficiently discussed on the Thai Embassy / Consulate websites. The features and benefits I point out - plus how it all works are not discussed at all on those Thai websites. They simply say - 12 month visa...
Many people do not have the finances in their home countries. Or in many cases people from Western countries come to Thailand on a Tourist Visa or Exempt Entry, obtain a 90 Single Entry O Visa, then convert to a 12 month Extension of Stay. Then do the Extension every year. It accomplishes about the same thing. The O-A is just a neat and tidy package.
Non Immigrant O-A Multi Entry Visa - one year - nearly auto extended for another 12 months with an Exit and Reentry to Thailand. Applicant must be age 50 or older. The application must be done at a Thai Embassy / Consulate in one's home country. There is a medical and police background check (nothing major). Finances are: Have to have 800,000 Thai Baht equivalent in a bank account outside Thailand - usually in one's home country. Other details can be found at the Thai Embassy website in one's home country.