What are the benefits of applying for an OA retirement visa versus an O retirement visa in Thailand?

The benefit to OA is you never have to transfer money to Thailand or show money in a Thai bank account.

It's a 1 year multi-entry visa so each time you enter Thailand during that year, you're stamped in for another year (as long as you have a year of the required insurance).

So if you leave and re-enter just before it expires, you get 2 years out of the visa without showing money in Thailand. Then you can go home and apply for a new OA for the next 2 years. Rinse and repeat.

For a non-O annual extension in Thailand you have to show the 800,000 baht in your Thai bank account and it's going to be locked up for half the year and can never drop below 400,000 the other half of the year. You lose access to this money and any potential it has. On the other hand, the non-O doesn't require insurance.

if you're INSIDE thailand you can't get an OA so if you're here your choice is made for you

Apply for the 90 day Non-O, wait the under review period, go back get it inked in, wait the 2 months go back and get the year extension..

Remember you need a MINIMUM of 15 days left on your current visa exempt entry stamp or the 30 day extension from it to apply for the in country Non-O at Chaengwattana Section C-1 (visa issuing section) because the applications goes under review for 14 days before you go back to get the Non-O and new 90 day stamp inked in.

Also your only option to get that visa (and your first yearly extension) as a US citizen is the 800K baht banked money method. You need to show the 800K baht was transferred into a thai bank account in your name only from abroad to qualify for using it as proof of funds for the Non-O at Chaengwattana.