Ok all you experts out there:

I have been in TL now for 3 years on a O A Retirement visa and have had extentions based on 800K in the bank, however this year I have been receiving medical treatment and the cost of this has made it impossible for me to keep the 800K in the bank for the 3 months before the extention is due.

I need to stay in TL for another 3 months after the date of my next extention so will only want a 90 day extention after that I will leave TL for about a year.

Even though my treatment will be finished I was looking to apply for an extention on medical grounds with a letter from the doctor saying I needed to stay for check up's. I see in the new extention regs that you now have to prove you are unfit to travel as well as being in need of treatment. This would mean that in my case an extention would not be granted.

My question is: What is my best option to get a 90 day extention? assuming they dont change the law again.

After that I would let the O A lapse and when I come back would apply for a new one.

You are on a retirement extension not a O-A.

Without knowing anything about you (married etc.) all I can think of is to go to a Thai Consulate in a neighbouring country and get a Tourist Visa (60 days) and extend it at Immigration for a further 30 days.

Note that you can also use an income of 65,000 a month or a combination of income and money in the bank totaling 800,000. In the last case the money doesn't need to be in the bank for 3 months.

Ok some more info

I am not married

I do not have 65k per month income, nor do I have 800K in the bank.

I dont need the expense of going to another country to apply for a tourist visa if I can help it.

I do not want to continue with a retiremeny visa but only to extend my stay for another 90 days before I leave the country for about a year.

Sorry, but post no.2 is it.

I think I would try and get the doctor to write something up or at least talk to immigration about it.

Thailand has been pushing medical care as a reaon to come here so you might find immigration to be a little flexable.