Can South Africans travel directly from Namibia to volunteer in Thailand, and what visa do they need?

A South African citizen invited to volunteer in Thailand is inquiring whether they can travel directly from Namibia or if they must first return to South Africa. They also seek information regarding the visa needed for volunteering. Several comments indicate that a specific visa, likely the Non Immigrant O visa based on volunteering, will be necessary and that the inviting organization must provide the required paperwork. Moreover, there are indications that certain visas may face restrictions related to regions such as Africa and India.