this really varies from Amphur to Amphur. I've just been through it in Hua Hin and it was...challenging. Not saying it's difficult, but they insisted on a new copy of my passport stamped as authentic by the British Embassy in Bangkok (I had already done this once to get married and had colour copies), then translated and certified at the Ministry of Foreign Affairs. Once they had these documents they sent us away for 5 weeks until they had an appointment available. When we returned for our appointment they asked us to sign a few copies of documents and sent us away for another week whilst the Sheriff checked everything was okay. A week later we waited for 2 and a half hours for them to issue the yellow book and pink ID card. So, all in it probably took me about 2 months to get the yellow tabian baan, when other people seem to get them with no problems. Before anyone says it, yes we go smartly dressed and are always polite.
I am unsure if that will work (not saying it won't). I would ask another question to check it is possible to do this and the moderators will confirm. Good luck.
Are you certain you want the OA visa and not the non O?
The non O requires 800,000 baht in a Thai bank for the first year, but otherwise it's a simpler option as you don't need the criminal checks or the insurance policy from one of only a few companies.
Get a non O retirement visa online from the Thai Embassy in your country before coming to Thailand.
Open a bank account in Thailand as soon as you get here and deposit 800,000 baht. Wait 2 months and extend your stay at immigration for 12 months based on retirement.