Do I need proof of onward travel when applying for a Non-Immigrant O visa for Thailand?

okay, let's try to get you back on track

you say;

𝗜 𝘄𝗶𝗹𝗹 𝗯𝗲 𝗮𝗽𝗽𝗹𝘆𝗶𝗻𝗴 𝗳𝗼𝗿 𝗮𝗻 𝗻𝗼𝗻 𝗶𝗺𝗺𝗶𝗴𝗿𝗮𝗻𝘁 𝗢 𝘃𝗶𝘀𝗮 𝘀𝗼𝗼𝗻 𝘄𝗶𝘁𝗵 𝘁𝗵𝗲 𝗶𝗻𝘁𝗲𝗻𝘁𝗶𝗼𝗻 𝗼𝗳 𝗰𝗵𝗮𝗻𝗴𝗶𝗻𝗴 𝘁𝗵𝗶𝘀 𝘁𝗼 𝗮𝗻 𝗢-𝗔 𝘃𝗶𝘀𝗮 𝗮𝗳𝘁𝗲𝗿 𝗜 𝗮𝗿𝗿𝗶𝘃𝗲 𝗶𝗻 𝗣𝗵𝘂𝗸𝗲𝘁.

Just so you are clear you can't apply for OR change anything to an O-A visa after you get to thailand. You BUY an O-A visa from the thai consulate in your home country before you wing your way here. It's the visa that requires the police background check, medical certificate and proof of mandatory health insurance.

So with that out of the way you That leaves you with two choices ;

Either get a 90 day single entry Non-O visa before you come here <- by meeting the requirements for it at the thai consulate where you would apply for it

OR

Get a single entry tourist visa from the thai consulate, come here, get your bank account open, transfer the 800K baht funds in from abroad THEN apply for the in country Non-O visa at the immigration office for 2000baht and after 2 months when the funds are seasoned you can apply for the year extension based on being over 50 (retirement) for 1900baht.

I fly Emirates and am on Non O. They don’t ask for onward ticket.

General information for retirement extension inside Thailand who is already on Non-O initial visa issued at the embassy/consulate or the immigration office.

What reason for Non-O are you seeking?

In any way, you cannot apply for Non-OA in Thailand (Non-OA is available only at the embassy/consulate in your home country). You can apply for a 1-year extension for the same reason as your Non-O visa you would enter Thailand with.

If you have a Non-O visa beforehand, you wouldn't need and the airline doesn't check an outbound ticket. Still, you can check with your air carrier about the latest policy. Or try other Thailand travel-related groups for specific airline's current policies.