What are the requirements for extending a Visa OA for a retiree in Thailand?

People on an OA visa can get two years of stay out of a visa valid for a year by exiting/re-entering before the VISA itself expires (and showing they have another year of health insurance).

So if the visa itself is still valid (NOT the tiny red year entry stamp she got when she entered) she could exit/re-enter and get another year (or as long as her health insurance is good for)

Now if you want to go the year extension route inside the country at the immigration office; I would also contact your Belgian Embassy here to see if they offer the affidavit of income from abroad notary letter <- which you can use in lieu of banking the 800K baht in a thai account in her name only for 2 months before she applies for an extension.

Either way you go with it she will need to show she has the 3 million baht general health coverage now because the requirements changed on Oct 1st 2022 from the 40k/400k out/in-patient coverage to 3 million baht general coverage.