Can I activate a Non OA visa in Thailand if I applied for it from the US?

Hope i have this right, apologies if not.

I am going to make an assumption that you are entering on basis of retirement?

I would drop the OA idea and just go for Non-O based on retirement when you arrive which can be done in country.

The challenge you will have is with the first entry you get 30 days (plus one 30 day extention)

to start that process of 90 day non-o based on retirement.

If you have all the paperwork should be relatively straight forward.

The OA requires longterm insurance, non-o doesn't.

Your challenge will be to get setup and get your money situation sorted for your year extension.

It really shouldn't be difficult if you affairs are in order.

It cannot be activated inside of Thailand.

Once you get it approved you will have to depart and reenter Thailand to activate it.

You may wish to ponder a tourism trip to Cambodia sometime in the OA visa first year. Or Singapore if it opens.

No