@Paulo *********
If you enter Thailand on a tourist entry you can then apply for a 90 day O visa. At the end of the 90 days you then apply for a 1 year extension of that O visa. There is no insurance requirement when you apply and extend inside Thailand. Most retirees are here on a non-O and extend yearly.
In my own case, I switched from my OA to a 10 year LTR VISA. Both of these require insurance so the switch was easy for me.
It is possible you need insurance for an O visa if your purchase it from the Thai consulate in your home country. You won’t need insurance to extend it inside Thailand.
The ultimate goal is to get on yearly extensions unless you plan to keep returning to Australia to buy a new OA every 1-2 years.