@Paul *******
Sorry for not getting back to you sooner. The "O" class of visa is "other", and covers a lot of different categories. There's two categories of "O" visas that is being talked about:
Non-O on the basis of being over 50/ "Retirement". For this visa, you will need to have 800K THB in a bank account in your home country, and be over 50.
Non-O on the basis of being a trailing spouse. For this visa, you'll need to be legally married to someone who is getting a Non-O Visa on the Basis of Being Over 50/ "Retirement".
The Non-O visa based on retirement is ALSO sold inside Thailand. The Non-O visa based on being a trailing spouse for someone who is on a retirement visa is NOT sold in Thailand.
If person A, the husband, enters Thailand on a Non-O visa based on retirement, and person B, the wife, enters Thailand on a Non-O visa based on being a trailing spouse, only 800K THB is needed in a Thai bank account, seasoned for 2 months. There is NO financial requirements for a one year extension of stay based on being a trailing spouse.
If person A, the husband, and person B, the wife, enters Thailand on a tourist visa, both person A, and person B would have to buy a Non-O visa based on Retirement inside Thailand. This would require 1.6M THB in a Thai bank account.
I hope that answers the question.