- Narcolepsy - Rare sleep disorder characterized by uncontrollable episodes of falling asleep at any place or time.
- Nasal polyps - non-malignant growths in the nasal cavities, usually in both sides of the nose.
- Nasal septum, deviated - Crookedness or other abnormality of the septum, the structure dividing the nose in 2 equal parts.
- Nausea and Vomiting - Nausea is usually described as queasy, unpleasant sensations in the stomach leading to the urge to vomit.
- Nephritis, Interstitial - is an inflammation of the connective tissue of the kidney.
- Nephrotic syndrome - a form of chronic kidney disease beginning in childhood.
- Nystagmus - is an uncontrollable (involuntary), repetitive, rhythmic movement of the eyes.
- Nevi - Collections of pigmented cells called melanocytes that appear on skin. Most are present on the torso but are also commonly found on the face, arms and legs.
- Nosebleed - Loss of blood through the nose. Excessive nose picking is a common cause of nosebleeds in children.
- Obesity - A condition of excess body weight. May be defined as over 25% body fat in females and over 20% body fat in males.
- Obsessive-compulsive disorder - disorder characterized by recurrent, intrusive thoughts and repetitive, ritualistic behaviors.
- Optic neuritis - is an inflammation of the optic nerve causing sudden visual loss in part of the visual field.
- Osteoarthritis - Degeneration of cartilage at a joint and growth of bone "spurs" that inflame surrounding tissue.
- Osteomyelitis - Infection of the bone and bone marrow. It can involve any bone in the body.
- Osteoporosis - Loss of normal bone density, mass and strength, leading to increased thinning and vulnerability to fracture.
- Oral cancer - Cancer of the mouth, lip and/or oropharynx (part of the throat at the back of the mouth). Most begin in the tongue and in the floor of the mouth.
- Osgood-schlatter disease - Inflammation of the tendon where the kneecap meets the top of the shinbone (patellar tendon). It causes swelling and tenderness.
- Ovarian cancer - Cancer that begins in the cells that compose the ovaries, including surface epithelial cells, germ cells, and the sex cord-stromal cells.