That doesnât sound like an O-A visa. An O-A is 1 year multiple entry. A friend of mine got a 2 or 3 month in the US and then had to convert to a yearly extension of stay based on being retired (i.e. over 50)
I think it is active as soon as you get so you would lose a month. The main thing is to give yourself 1 years time to get your financial requirements in order if you are planning to use the monthly income method when you apply for your year extension. Losing a month will hardly matter. If you are planning to use the 800,000 baht banked money method then it matters even less.
Google criminal background check for your city. Some people in the US get the check from their local police department. I submitted my fingerprints and a form at a UPS office and got the results mailed to me a week later. You only need to do the criminal background check in the US for an O-A visa. Medical report in the US too. Just ask your doctor to sign the form. Your money to meet the financial requirements can be in a US bank. Pick a Consulate in the US and look at the requirements on their website. I used the LA Consulate.