I am not sure if immigration require the 800k to be verified as freshly arrived from overseas to get the O-Visa.. I have heard of that. If you have plenty of money, an agent can take care of all the steps for you ...(I even read of it being done without leaving Thailand.) Some agents charge a good amount for all of the steps. (Obtain O Visa, then retirement extension, total 15 months stay).
There is an agent on Soi Post Office does a cheaper price and fair service. If you have the 800k sitting there the price is much less.