for O visa you need to have a large sum in Thai Bank but it gets no interest. With OA visa, you don't need a large amount of cash in a Thai bank, so that money can stay in Australia and earn interest. Either way you need that money in a bank account to meet Non O or non OA visa. So if you want health insurance and to earn interest on your lump sum required for either visa, then get OA. If you don't mind losing a bit of interest by having that lump sum in a Thai bank and much cheaper insurance then go O visa. Many people convert from O to O as the approved insurance policies are very expensive and possibly it'd be cheaper overall to get O visa. Australia doesn't offer O visa, only OA. O you get within Thailand.