Hi. I fill out the form for a Non-immigrant (O) visa and there is information at the bottom:
Once in Thailand, holder of non-immigrant visa can apply for an extension of stay to 1 year from the Immigration Bureau.
How to do it?
TLDR : Answer Summary
To apply for a 1-year extension of stay on a Non-Immigrant (O) visa in Thailand, visit your local immigration office and ask for the specific requirements, as these can vary by province. Generally, you will need a Thai bank account with a balance that ranges from 400,000 baht to 800,000 baht.