I hold an O-A (Retirement) Visa. The expiration date on the visa is Sept. 19, 2011, one year from the application date. I have been in Thailand since last Nov. 5. I recently left Thailand for a one week stay in Hong Kong. When I returned the immigration officer stamped my passport admitted until 2 Mar. 2012. Can I return home to the USA and return without renewing my visa as long as I return before 2 Mar. 2012? Or do I have to renew my visa prior to 19 Sept. 2011?

Any return after original expiration date of visa (Sep 19) will require a re-entry permit to keep the then current stay alive. You can leave again and return on Sep 18 to get a new entry until Sep 2012 so would not have to start extensions until that date if you wish.

As said, take another trip and return before 19 September 2011 or get a re-entry permit and you are good to go until 2 March 2012. Just prior to March 2012 you can apply for a 1 year extension of stay based on retirement from immigration, if you meet the financial requirements, there is no need to get another visa.