For your first extension, you will have to use the 800,000 method. Normally two months before applying and 3 months after.
Some people have reported that some Bangkok Bank branches now require the funds to be deposited for longer than two months, otherwise they will not issue you the certificate.
Please check with your local Immigration Office as well as your local bank branch.
Just go to your local Immigration office once you get here (with your No O obtained from overseas) and ask them what their requirements are for an extension. The requirements vary from office to office.