Hey Nam, I just went through this process and I’m currently on the non-OA visa. I arrived in Thailand a few weeks ago.
If your partner chooses to go with the OA, he would provide bank statements for the previous three months in his home country showing “the equivalent” of ฿800,000 in his home currency, never dropping below that amount in those proceeding three months. It’s perfectly acceptable.
But once your partner arrives in Thailand and gets an apartment, he can go to immigration with his lease and TM 30 form that his landlord will provide, and get a certificate of residence. That COR, coupled with a non-OA visa and a few other documents will allow him to open a Thai bank account. If your partner’s plan is to stay in Thailand beyond a year and get an extension of stay for his non-OA visa, he would need to put ฿800,000 in a Thai bank account “no less than two months before the annual renewal” and it must remain in that Thai bank account never going below ฿800,000 for “three months after the renewal.” The benefit of the non-OA is that you get the visa from your home country, and can use your home countries bank account, “but only for the first year”, but as you’ve also seen on this board it does come with extra requirements. Health insurance. Criminal background check in your home country, etc.
So you’re going to want to have your partner decide what his long-term plans are. If he plans to stay in Thailand and renew every year, the non-O is a better option long term and with easier annual extensions because it doesn’t require health insurance or criminal background check. However, there are some other steps that are involved in it upfront.
For instance, if he wanted to go with a non-O instead of the non-OA, he would apply through the eVisa site for the non-O (90 day) in his home country. That basically allows him to get into Thailand for 90 days to do a few other steps before he’s allowed to extend it in additional 12 months.
Upon arrival, he would sign a 12 month lease, and go to immigration  with his lease and TM 30 form from his landlord to get the certificate of residence. With that COR and his non-O (90 Day) visa, he could then open a Thai bank account as soon as possible to deposit the ฿800,000 into his new account immediately. 2 months after the deposit, the money would be considered ‘seasoned’, and he could then go and extend that non O visa at immigration for an additional 12 months (ideally in the last 30 days of his current 90 day visa).
But remember, the balance of that Thai bank account would need to remain at 800,000 for the three months after the extension is approved.
Then for the rest of the year money from that account could be used, but it cannot go below ฿400,000 for the rest of the year until two months before his annual extension when he’ll have to add money to bring it back up to ฿800,000 again to be in compliance for the extension of stay of his non-O visa.
So basically it’s the best to think about ithe 800k as a needing to be locked up for five months out of the year to be in compliance for his extensions of stay (with this method.)
There are variations on all of this as you’ll see different people chime in, but that is what my homework gave me on these boards and through other research. Good luck.