Must the OA be obtained from my home country, and can a non-resident apply elsewhere?

Start with the upfront fact: the Non-Immigrant 'OA' Retirement Visa is issued entirely and solely through the Thai Embassy or Consulate system outside of Thailand. From there, the key constraint is that the vast majority of Thai Embassies and Consulates restrict who they will accept OA applications from. As a rule, they want to see that you are either a citizen of the country where that Embassy or Consulate is located, or a legitimate permanent resident or long-term legitimate visa holder there. You generally cannot apply through an Embassy in a country where you are merely passing through.

This matters more than it first appears, because of how often the situation comes up. People frequently arrive in Thailand as a tourist or under Visa Exemption status, settle in, and then decide they want an OA. First, they have to leave Thailand to apply at all. But here is the trap: they cannot simply pop over to Vietnam and apply through the Thai Embassy system there. The Thai Embassy in Vietnam requires OA applicants to hold a Vietnamese Permanent Residence Card or Vietnamese citizenship. A tourist in Vietnam does not qualify. The application has to go through the country where you are a citizen, a permanent resident, or hold equivalent long-term standing.

It is also worth knowing that these proof-of-location rules are stricter for the OA than for the Non-Immigrant 'O' Retirement Visa, and the reason is the OA's criminal background check requirement. Consider an American couple trying to apply through the Thai Embassy in Vietnam. Vietnamese authorities cannot produce a valid police clearance certificate on two American citizens who have no long-term status in Vietnam. They simply are not in a position to run that background check. So that couple would need to apply through the Thai Embassy system in the United States, where they can obtain the required FBI background check from within America. The background check requirement effectively anchors the OA application to a country where the applicant genuinely belongs.

Each Embassy and Consulate sets its own specific requirements, and those requirements vary by visa type as well, so this is exactly the kind of detail to confirm early, because it shapes not just your eligibility but your travel and timing plans. For the precise rules at the specific Embassy you intend to use, raise it during your eligibility assessment so we can confirm the exact conditions for your situation.