I am on a tourist visa after letting my O-A visa expired by mistake. I want to go to Savannakhet to get a NON O visa and later a year extention in Thailand ( retirement ).. In Savannakhet what documents do i need ?. I will go to BKK next week to obtain the Income letter from the embassy ... What else do i need ? Thanks to all. Have a nice day

I'll move your question to the, more appropriate, Thai Visa section.

Savannakhet is not a good location to get a non-o visa based upon being 50 or over. They are reluctant to do them and have denied them to many people.

Vientiane is the best place. You will just need your passport and the income letter. Just attach a copy of the letter to the application and show them the original so you can use it to apply for your extension. There have been reports of them asking for proof of residence. Anything to prove your address will work if needed.