I just received a multiple entry O-A visa (based on retirement) from the Los Angeles consulate for $200 with an "enter before" date of May 3, 2016. I want to make sure I understand my situation:

1) I will enter Thailand on June 21, 2015. My understanding is that I will receive an "admitted until" stamp of June 20, 2016. Correct?

2) If I leave Thailand and return before May 3, 2016, I do not need a re-entry permit, and upon arrival will get a new 1-year stamp. For example, if I travel to a neighboring country in December 2015, and return say on December 18, I will get a new stamp with an "admitted to" date of December 18, 2016. Correct?

However, I will need (for business reasons) to return to the US from January 2016 through approximately May/June 2016. I won't be able to return by May 3, 2016, so I will need to apply for a new O-A visa at the Los Angeles consulate before returning to Thailand in June 2016, correct?

BTW, getting the visa was pretty easy: quick stop at my local police to get proof-I'm-not-a-criminal-form, a couple of emails to obtain proof of income, a 5 minute visit to my Doctor's office to get a medical certificate (no exam needed). Then a quick trip to the notary and the rest done by mail. It all seems easier than doing an extension of stay in Thailand, and no worries (for at least 1 year) about re-entry.

Anything I am missing?

Thanks....

1 and 2 are correct.

You can get a re-entry permit before you leave in January to keep the permitted to stay date you have at that time valid when you return. Using your example in two that would Be December 18th 2016.