@Glenice ********
The OA cannot be obtained inside Thailand.
The Non O Visa as a retiree can be obtained inside Thailand.
You will have to meet the requirements for the Non O Visa by having 800,000 Thai baht on deposit in a Thai bank account in your name only.
You can arrive into Thailand Visa EXEMPT and get 30 days stamp plus one 30 day extension of stay at Thai immigration office.
When you first arrive, visit your local Thai immigration office and ask for their list of requirements for the Non O Visa as retiree and the One year extension of stay as retired.
Also you could obtain the Non O Visa as retiree from your Thai Embassy or Consulate in Australia. Then will get you a 90 day stamp on arrival into Thailand.