When can I apply to extend my Non-Immigrant O visa for one year in Chiang Mai, and what documents do I need?

swing by the Chiang Mai immigration office at the airport and get the hand out from them that lists the requirements

This is just a rough list of what you will have to show;

- Bank letter from the bank

- bank book updated showing 800K baht in a thai bank account in your name only for 2 months before you apply

- Lease

- TM30 receipt

-filled out Tm7 application for extension

-copies of the data page of your passport, your Non-O visa sticker, your entry stamp and your TM6 departure card

- map to your house

I'm sure there's a few other things which is why I said go get the hand out that lists the requirements up in Chiang Mai

how about stating for what reason you will be extending?