I'll stay in Thailand for 6 months for volunteer work. But the consulate at Hong Kong only gives me a non-immigrant O 3-month visa.

Do I extend it at the nearest immigration office in Thailand a couple of days before the visa expires? Will it be a 3-month extension? Or do I need to visit a consulate outside of Thailand? What documentation is needed?

I'll stay in Thailand for 6 months for volunteer work. But the consulate at Hong Kong only gives me a non-immigrant O 3-month visa.

Do I extend it at the nearest immigration office in Thailand a couple of days before the visa expires? Will it be a 3-month extension? Or do I need to visit a consulate outside of Thailand? What documentation is needed?

Just a quick comment, I assume the Thai office understands you are going to do bona fide volunteer work. So, why do you bother if they obviously don't consider your help as wanted or needed. There were similar problems after the sumani where some volunteers were put through the wringer. They would appreciate you more if you just send them cash and stay where you are.

I'll stay in Thailand for 6 months for volunteer work. But the consulate at Hong Kong only gives me a non-immigrant O 3-month visa.

Do I extend it at the nearest immigration office in Thailand a couple of days before the visa expires? Will it be a 3-month extension? Or do I need to visit a consulate outside of Thailand? What documentation is needed?

You will have to go outside Thailand and apply for another Visa at a Consulate

I'll stay in Thailand for 6 months for volunteer work. But the consulate at Hong Kong only gives me a non-immigrant O 3-month visa.

Do I extend it at the nearest immigration office in Thailand a couple of days before the visa expires? Will it be a 3-month extension? Or do I need to visit a consulate outside of Thailand? What documentation is needed?

Just a quick comment, I assume the Thai office understands you are going to do bona fide volunteer work. So, why do you bother if they obviously don't consider your help as wanted or needed. There were similar problems after the sumani where some volunteers were put through the wringer. They would appreciate you more if you just send them cash and stay where you are.

What a useful comment !