Are you certain you want the OA visa and not the non O?
The non O requires 800,000 baht in a Thai bank for the first year, but otherwise it's a simpler option as you don't need the criminal checks or the insurance policy from one of only a few companies.
Get a non O retirement visa online from the Thai Embassy in your country before coming to Thailand.
Open a bank account in Thailand as soon as you get here and deposit 800,000 baht. Wait 2 months and extend your stay at immigration for 12 months based on retirement.
you need to get a non O visa before travelling to Thailand online from the Thai Embassy in your country. It used to be possible to come without a visa and open a bank account, but that's not possible now.
If you avoid the non OA visa, you don't need criminal checks and medical insurance.
It's more complicated than getting a non O visa before you come to Thailand, as it's virtually impossible to open a bank account without a long term visa.