If my non immigrant o visa is stamped expiry 1 april 2012, does that mean I cannot enter thai9land the first time after 1 april 2012, or that I can enter thailand the last time 1 year after my first entry into thailand?

After the expiry date you can no longer use the visa to enter Thailand.

It means any/all entries must be made by April 1, 2012. Non immigrant visas are either single entry valid for 90 days from date of issue or multi entry valid for one year from date of issue.

If multi 0, do a run the day before expiry and get another 90 days.

thanks.

I thought that the visa meant that I could enter thailand the 1st time , up to a year after the visa's date of issue, and that If I entered thailand within a year of my 1st entry, I would be able to extend my stay for another 3 months.

So I was wrong, and my year's visa started the day the visa was issued.??

Start on date of issue.

Last date you can enter Thailand with the visa would be April 1, but 1 day before is better (safer)