Do I need to inform immigration of my travel date after getting a single entry retirement visa in Thailand?

After obtaining a single entry permit on your O retirement visa, you do not need to notify immigration of an exact date before departing. As long as you possess a re-entry permit in your passport, you can travel freely as long as you return before your initial entry stamp expires. Remember, the single re-entry permit allows for one exit and re-entry; a second exit will require purchasing another re-entry permit.