If you are at least 50 years old, still healthy and if your income is sufficient to live there, it will be easy to stay in Thailand permanently. You will have to prove an income equivalent to 65,000 baht per month (about 1600 €) or dispose of 800,000 baht (20,000 €) on a bank account in Thailand. This bank account can be a savings account. The 800,000 THB amount must be on the account for more than 60 days prior to the application, but after this, you can use that money to live on it. The visa will be issued for one year duration. You will have to justify again the 800,000 baht amount three months before the annual renewal. A combination of both solutions is possible, so if you have 400,000 baht (about € 10,000) on an account and a monthly income of 35,000 baht (about 875 €), you can get your O-A retiree visa as well.

The demand can be made at the embassy of your current country of residence, or directly at the Thai immigration office. Appart from the income attestations, 
you will need a passport valid over six months after the end of your stay (18 months in practice), three photos and the visa application form in triplicate. It will cost you 2000 baht (about 50 €) for a single entry visa and 5000 baht (about 125 €) for a multiple entry visa. Retiree visa does not allow you to have a business in Thailand.