Do I Need to Show Funds in a Thai Bank for an O-A Visa Extension?

You have a lot of time to decide what to do by January 2021. Including getting a new Non O visa and extend it inside Thailand...no health insurance required. But 800,000 Thai baht in Thai bank is required. There is a TOPIC Conversion visa exempt to Non O ....read it for helpful how to advice.

If you choose to extend it inside Thailand then you need 800,000 Thai baht banked in Thailand and health insurance to meet requirements.

When you entered Thailand on an OA visa you should have received an entry stamp with permission to stay until one year in the future. That permission to stay doesn't disappear when your OA visa expires, just your ability to leave the country and re-enter the country using that OA visa without needing a re-entry permit. After your OA visa expires should you wish to leave and return you would need to purchase a re-entry permit. However, no foreigner (with very specialized exceptions) can enter Thailand at the moment so it's a moot point.

To answer your question directly you should be good to stay until next January (you should have an entry stamp with this permission to stay stamp in your passport). Should Thailand open its borders before then and you want to travel and return to Thailand you will need a re-entry permit and meet all the additional qualifications there may be in place at that time (health certificate, mandatory quarantine period, etc.)

Probably O-A, no? I think it is the best for long stay. After 30 years I am not going to live anywhere else if I can help it.

I plan to retire in Thailand after having lived here for 30 years already but I have an excellent international health insurance (Cigna). Surely I would not have to apply for a Thai health insurance as well, no?