I am in the last step in the process for my retirement visa. I submitted and had accepted a six month agreement for my hotel room. The landlord has to be cooperative as they require landlord's id, etc. Each immigration office can set their own rules, so go to the office and get the list of requirements.
I am in the process of doing this myself. if you already have health insurance, by one of the approved insurance companies, I would apply at an embassy out of the country. The problem coming in with a tourist visa and then converting it to an OA is the opening of the bank account. some people say you can go to the immigration office, get your certificate of residency and open a bank account with this. I ended up hiring an agent and having to use a specific bank that charged me for insurance that I felt was a poor value. The total amount spent was about 10,000 baht or $300. Shop around as the costs for hiring an agent vary widely, with the money transferred to my bank account I then went to the immigration office and had their volunteers help me fill in all the paperwork which was turned in last week and accepted by my local immigration office. It appears as though I'm at the stage in the process by which we are taking the steps bringing the proper papers again and following directions of the immigration officers, pretty straightforward, good luck.