Arnold, the situation is different depending on exactly what visa you have:
(1) Non-Immigrant O-A visa
obtained in your home country prior to entry into Thailand
(2) an extension of an existing visa for purpose of retirement in Thailand,
obtained in Thailand
In the first case (O-A visa issued in home country), you are entitled to a one-year permission-to-stay stamp each time you enter Thailand. After the last entry into Thailand prior to the expiration of the O-A visa and you are granted a permission-to-stay for one-year from that date of entry (even though it may be way beyond the expiration date of the O-A visa), to "keep the permission-to-stay alive" and be able to exit/re-enter Thailand after the exipiration date of your O-A visa, you'd need a re-entry permit. Example:
- O-A visa issued in home country 01 November 2006, expires 31 October 2007.
You leave Thailand and re-enter on 30 October 2007, and are stamped with a permission-to-stay until 29 October 2008.
If you do NOT obtain a re-entry permit before your next trip out of Thailand, you need a new visa to re-enter Thailand.
If you DO obtain a re-entry permit before your next trip out of Thailand, you can re-enter on current visa/re-entry permit and stay until 29 October 2008.
In the second case (retirement extension issued in Thailand) your re-entry permit expires concurrent with the expiration of the visa, so you need to get both (a new extension and new re-entry permit) each year.
So, if you get an O-A visa in your home country, you can actually stretch it to (almost) two years without getting a new visa. That's the bonus of it. The downside is you need to go home to get it, and need to submit to police and medical clearance.