You cannot obtain a non O-A in Thailand.
You can arrive in Thailand with SETV or visa exempt and obtain a non O retirement and subsequent 12 month extension.
Requires 800k in Thai bank in your name on the day you apply for non O retirement.
Any flight requirements mentioned in your OP is onward flight not necessarily return flight.
You also ask this....
" If so will I be allowed to open a bank account to deposit the 800,000 Bht"...
If you arrive with a non O you will be able to open a Thai bank account.
If you arrive with tourist visa or visa exempt with intention to obtain non O retirement at Thai immigration, you would be best to use agent to assist with bank account.