@Jeff ********
a retirement "extension" can only be obtained in Thailand, but one must obtain the retirement 'visa' first. I suspect that you've just got confused with your terminology there.
There is a "Non-OA" Visa, a 'retirement' visa, which can only be obtained from one's own home country, which does require health insurance and Police Clearance among other requirements. A 'standard' (my choice of word) Non-O Visa based on being ''over 50'' that can be obtained at Thai Embassies or Consulates before arriving in Thailand that don't have the requirements of the 'OA' Visa.
Purchasing a Non-O outside of Thailand is necessary now, as once in Thailand, foreigners cannot open Thai bank accounts on Tourist Visas or Visa Exempt.