What Do I Need for My O-A Retirement Visa Extension at Hua Hin Immigration?

500 baht a day fine up to 40 days (caps out at 20K baht max)

IF you overstay over 90 days but under a year and turn yourself in (go to stamp out, go to the immigration office or go to fly out) you will get banned for 1 year, over 1 year but under 3 years banned for 3 years, over 3 years but under 5 years banned for 5 years, over 5 years banned for 10 years

BUT

IF you are CAUGHT OUT inside thailand on an overstay (of even a single day up to 1 year) you CAN be banned for 3 years. :O

I am not trying to scare-monger, I'm relating how the penalties are written

It is not so much the overstaying that is dangerous, it is running amuck for some other reason while you're on overstay and getting caught in the country on it

you get a letter from the hospital or doctor, and you have someone with signed power of attorney GO apply for a medical extension for you BEFORE you run on overstay.