Can I deposit the required 800,000 THB for an O-A visa in cash at a Thai bank, or must it come via bank transfer?

As far as I know, the money doesn't have to be in your home country. It can also already be in the Th account. However, it should have gotten there with a bank transfer in case immigration checks it at the later Eos. This is my information

For extensions off a Non O or a Non OA do not require funds (banked money) to be seen as coming from abroad. You just need the funds in your account.

For the initial Non O obtained in country some offices require it to be from abroad but some don’t care either.

If you want to use the 65k per month method then you need to show they come from abroad.

I assume you mean the 800,000 required to extend your stay for 1 year. You have already been on this visa or permission to stay for 1-2 years.

I move my money from my US bank to my Thai bank with wire transfers. I got my first extension of stay 18 months after I first entered Thailand on my OA visa. I moved the 800k during that 18 months. I don’t recall anyone asking about where it came from. I gave them a bank statement and I am pretty certain none of the wire transfers were coded as from a foreign source because typically they were coded as from a domestic source presumably having been routed through another Thai bank.

An OA visa is obtained in your home country from the Thai Embassy and showing money in your home country bank account.