- Obesity
- Obsessive compulsive disorder (OCD, anxiety disorder)
- Onchocerciasis (river blindness)
- Onycholysis
- Ophthalmoplegia (eye muscle weakness)
- Optic neuritis (optic nerve inflammation)
- Oral cysts
- Oral ulcers (mouth ulcers)
- Orbital cellulitis (soft tissue eye infection)
- Orbital fracture (broken bone near the eye)
- Orbital mass (growth near the eye)
- Orchitis (inflammation of the testicles)
- Osgood Schlatter disease (painful shinbone inflammation)
- Osteoarthritis (bone inflammation)
- Osteochondritis dissecans (joint disorder)
- Osteomyelitis (bone infection)
- Osteopenia (thin bone)
- Osteoporosis (extremely thin bone)
- Ototoxicity (ear toxicity)
- Ovarian cancer (female organ cancer)
- Ovarian cyst (female organ fluid collection)
- Overdose
- Ovarian torsion (twisting of the ovary)
- Overuse syndrome (injury from repetitive activity)