I extended an OA visa for 6 years at Chaengwattana in Bangkok. The financial requirements for a 1 year extension are the same for an O visa. The banked money requirement is 800,000 baht in your Thai bank account 2 months before you apply for a 1 year extension, 3 months after you apply and no less than 400,000 baht for the rest of the year. This was a change implemented 3-4 years ago. I needed the bank letter (good for 7 days at CW) affirming that I had a bank account and the balance, at the time of the letter, was recorded. I also needed a 1 year bank statement to show that I had met the yearlong requirements stated above, and I needed an updated bank book with a same day transaction.