In Chiang Mai you can apply for a new yearly extension when you have 45 days or less left on that stamp so in your case you can apply anytime after the 3rd of Nov.
You would need to have the 800K baht in a thai bank account in your name only for 2 months before you apply for the extension
AND
because you have an O-A visa you would need to have the mandatory health insurance coverage from one of the 14 thai approved insurance agencies with coverage for the whole next year.
Here's the list of them
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