Can you apply for a Non-immigrant O visa while already in Thailand?

It totally depends on the organisation you are volunteering with whether you can apply for Non-O volunteering inside Thailand at the local immigration office or you have to apply for one at the Thai Embassy/consulate in another country. Experience with a different organisation is irrelevant to your case. You have to talk with your organisation. They are the ones and only who can answer your question.

Here's a Report of obtaining the Volunteer Visa from the Thai Embassy in Hanoi Vietnam

You are ASKING about obtaining the Non O Visa as Volunteer with a NGO National Governing Body which will sponsor you for it and the Thai work permit that is also needed.

CORRECTION...

THANKS FOR YOUR HELPFUL UPDATE

@Tod ********

IT'S AVAILABLE INSIDE THAILAND...

Your sponsoring organization AND Thai Immigration can approve of it being issued inside Thailand. Work closely with your NGO sponsor.

OR.. you apply pply for it and obtain it while in your home country with a Thai Embassy or Consulate using the Evisa online application system.

OR... with the sponsoring documents from the NGO Volunteer organization, you apply for it in a neighboring SE Asian country.

Then when you have it, you will apply for the One year extension of stay as a Volunteer inside Thailand at your local Thai Immigration office.

There are a few forum Members who have obtained the Non O Visa as Volunteer

Use the search 🔍 tool top of the forum page

ENTER Volunteer Visa

CLICK on it and all posts about it will pop up to read.

It's a visa which has specific requirements for it. Meet them with your NGO supporting documents and you will receive it.