I have since four years a retirement extension. This year il June 27th. I go to Europe end of April and come back in September. Is it possible come back on a 30 days stay and renew than my extension or do I need a single O???

The airline might not let you on the flight with a one way ticket and no Visa.

If you do make it to Thailand it is possible but it is much simpler with a Non Imm O Visa.

Possible - YES, if done within the first week of arrival. I would strongly recommend a non-immigrant "O" visa.

Edited February 19, 201016 yr by InterestedObserver

It is possible to go through conversion process from visa exempt entry to Non-O visa to extension of stay with at least 21 days remaining on your permission to stay, but you are more likely to have a problem with the airline refusing boarding without either a valid visa or ticket out of the country within 30 days of arrival. The cost of the conversion process alone is 2,000 baht, which roughly equates to the cost of a single entry Non-O visa, so is often more trouble free just to obtain single entry Non-O visa before travelling and apply straight for the extension within the last 30 days of your permission to stay.

Thanks all for fast reply. I know what to do now!