- Obstetrics
- Occupational Health
- Ocular Herpes see Keratitis
- Onchocerciasis
- Oncology see Cancer
- Ophthalmology see Eye Diseases
- Oral Cancer see Mouth Cancer
- Oral Diseases
- Oral Diseases & HIV
- Orbital Cellulitis see Cellulitis
- Orthopedics see Skeletal System / Bone Diseases
- Osteoarthritis
- Osteonecrosis & Fosamax
- Osteoporosis
- Otitis Media see Ear infections
- Otolaryngology see Ear, Nose, Throat
- Ovarian Cancer
- Ovarian Cysts