Can I transition from a Non-OA Visa to a Non-O Visa while in Thailand?

You need to do the monthly payments while on the same visa type to qualify. So bringing the money in on an OA, will not qualify you for the monthly payment method on a O.

You will not be able to switch at Hua Hin Immigration but you will be able to use monthly 65k transfer after the first year once you have a full year of transfers.

You can't just "switch" from OA to O. You actually have to leave Thailand, come back in on visa exempt, and then apply for the O at an immigration office. I know for a fact Bangkok Immigration allows the use of 12 x 65k transfers, but each immigration office has their own rules

You can't simply "switch" visas. You need to get off the Non-OA visa and start from scratch on the Non-O visa. You might have a chance of getting away with using your monthly transfers if you get the Non-O at a nearby consulate so you can use money in your home bank for that then try using monthly transfers for the extension but that's far from guaranteed

There may be some immigration offices that would allow you to do this, but there are likely more than won't.

When you enter Thailand with a non-OA visa, that means EVERY extension after that will also require the mandatory insurance. If you want to stay in Thailand long-term without going back home every 2 years to apply for a new visa, a non-OA is not a good option because you will have that mandatory insurance forever.

The only way to get "off" the OA and extensions is to leave Thailand without a re-entry permit after your visa expires, and then return as a tourist and apply for a new non-O visa (which does not have an insurance requirement). The non-O visa requires 800,000 baht in a Thai bank account or embassy certified income. The first 1-year extension also requires those things, and you cannot use bank transfers for either of them. There have been scattered reports of people with a non-OA visa leaving and starting over on a non-O visa, and the immigration office accepting the bank transfers, but it's incredibly rare and not something I would stake my chance to stay in Thailand on.

If you are fine with keeping the required insurance, then what you propose would be fine. You would not be switching to any non-O, you would just get a 1-year extension after your OA entry stamp runs out, using monthly transfers and would still require the insurance.

The more certain thing to do would be to come on a non-O visa, use the 800,000 baht in the bank for the first year, and make the monthly transfers during that year. After that first year you would then be able to show the monthly transfers for your next extension and then you could take all your money out of the bank and continue the transfers forever instead.