When can I withdraw the 800,000 baht after applying for the Non O visa in Thailand?

Consider getting the OA visa in your own country using money bank in your own country. That way you don’t even ever have to deposit to ฿800,000 into a Thai bank. Then during that year before the extension start banking the 65,000 per month.  the OA gives you two years..

The plan is good, that way you can get the initial year extension using banked money method and you will be transferring in the required 65K baht a month minimum to use monthly income method the following year for that extension

As far as the seasoning of the 800K baht banked money method works, you need to have the 800K baht in the bank for 2 months before you apply for the year extension. The 800K needs to stay in the account for 3 months after the extension is granted and then the balance cannot go below 400K baht the rest of the year <- so you can't pull that last 400K baht out of the account until after you apply for your following year extension based on monthly income by transfer method