O
- OAB
- Obesity
- Obesity, childhood
- Obsessive-compulsive disorder (OCD)
- Obstetric cholestasis
- Obstructive sleep apnea
- Obstructive sleep apnea in children
- Occult primary cancer
- Occupational asthma
- OCD
- Ocular albinism
- Ocular rosacea
- Oculocutaneous albinism
- Oculosympathetic palsy
- ODD
- Odontogenic tumors and cysts
- OHSS
- Olfactory neuroblastoma
- Oligodendroglioma
- OLP
- Onychomycosis
- Oppositional defiant disorder (ODD)
- Optic glioma
- Optic nerve glioma
- Optic neuritis
- Optic pathway glioma
- Oral cancer
- Oral candidiasis
- Oral lichen planus
- Oral thrush
- Orchitis
- Orthostatic hypotension (postural hypotension)
- Osteoarthritis
- Osteoarthritis, cervical