O
- Occupational Therapy Association of South Africa (OTASA)
- Occupational Therapy in Occupational Health (OTOH)
- Ocean Sailing Association of Southern Africa (OSASA)
- Oceanographic Research Institute (ORI)
- Ocularists Association of Southern Africa (OASA)
- Office of Health Standards Compliance (OHSC)
- Office of the Health Ombud (OHO)
- Office of the Ombud for Financial Service Providers (FAISOmbud)
- Office Professionals of South Africa (OPSA)
- Oil and Protein Seeds Development Trust (OPDT)
- Oil Seeds Advisory Committee (OAC)
- Onderspoort Veterinary Institute (OVI)
- Open Society Foundation for South Africa (OSFSA)
- Operation Hunger (OH)
- Operations Research Society of South Africa (ORRSA)
- Opthalmological Society of South Africa (OSSA)
- OR Tambo International Airport Special Economic Zone (OR TAMBO SEZ)
- Oral Hygienists Association of South Africa (OHASA)
- Orchid Society of Northern Transvaal ()
- Order of St John South Africa (OSJ-SA)
- Orff Schulwerk Society of South Africa (OSSSA)
- Organ Donor Foundation of Southern Africa (ODF)
- Organisation of Civic Rights (OCR)
- Organisation of South African Law Libraries (OSALL)
- Orientation and Mobility Association of South Africa (OMASA)
- Oudtshoorn Business Chamber (OBC)
- OUT well-being (OUT)
- Outdoor Power Product Association of South Africa (OPPASA)
- Outeniqua Canoe Club (OCC)
- Overberg Crane Group (OCG)