What should I know about applying for a Non Immigrant O-A visa and related requirements for proof of onward flight and accommodation?

Look for accommodation booked by the month. then use that

Book cheap ticket to Singapore or KL somewhere close

With a non-o 90 day visa with intent to retire. It's not Thai immigration that will give you a problem with an onward ticket, it's the airline. The Thai IMO that you meet on arrival will see that you have the CORRECT visa for your visit and stamp you in accordingly.

Airline protocol worldwide is if you're refused entry to a country it's their responsibility, and at their expanse to return you to where you departed from.

Airlines are not immigration officers and are not interested in your intent to apply for an extension to stay. It's not their responsibility to liaise with immigration. The airlines call the shots, it's wrong but that's the way it is. Have a safe and pleasant retirement 🙏

Just have an onward ticket of some sort booked. Doesn’t need to be a flight. Cheap train to Laos for example. Dont do what I did… and try to explain that I’m not sure when I’ll be leaving as I’m going to apply for a different visa once in the country. It won’t wash! Trust me 🤣

Do you realise a

Non-O is more suited to most

( Aussie living in Thailand )

Have you chosen a Non-OA for a reason ?

I used a one way ticket to Thailand. I also used an address of a house I stay in Thailand belongs to my family and was not asked for more than that. Was for 90 day Non-O

Don't mix up your visa types. The Non-OA is a 12 month visa that requires insurance, police clearance etc. It sounds like the visa you actually want is the Non-O visa which gives you 90 days inside Thailand but can be extended for 12 months at a local immigration office. As to the date just find a return flight within the 90 days and put in the flight details, no need to book. A few days accommodation should be fine

A Non-O gives you 90 days, The Non-OA gives you a year on entry