@Neal ******
I’ve heard others say that they have some type of Medicare supplemental insurance that they can use overseas. I don’t know the details myself. I only have Medicare Part A and I can’t use this outside the US.
I used to have an OA visa but I got it in 2017 before there was an insurance requirement. I don’t know what insurance is acceptable when getting an OA in your home country but I highly doubt Medicare or any supplement would qualify. I have Thai insurance, Pacific Cross. They want you to have insurance you can use in Thailand. Eventually you have to get 1 year extensions of your OA and then you will need Thai insurance to do that. To use non-Thai insurance for the 1 year extensions you have to get your insurance company to sign a document that says their policy conforms to the Thai requirements. Mostly I hear that insurance companies will not sign the form.
I would not count on Medicare helping you with regards to meeting any immigration requirements or paying for any medical treatment in Thailand. If you want health insurance in Thailand you need to pay for it yourself or self insure.