The best and most accurate information in my opinion comes from the various embassy Facebook pages. The Ambassadors are talking direct to the Thai Government and then post what they are told.
Everything else that you read on here is speculation, wild guesses or outright lies.
Pacific Health Insurance are on the approved insurers list for O-A visa holders.
They have a range of policies designed to meet the immigration requirements.
If you are just doing this to satisfy the visa application you can nominate a maximum deductible of 300,000 baht. This would reduce the annual premium to around 15,000 baht. I only know this because I have just done this with them myself.
You can go to immigration 30 days before your expiry date. This is probably a good idea for your first extension as it will give you plenty of time to correct any errors that you may have made with the paperwork.
Different conditions apply depending on what visa you currently hold and how you are justifying your financial position i.e. 80,000 baht in the bank or 65,000 baht per month or a combination of both.
The immigration offices also have different interpretations of the requirements depending on which office you go to.