Obesity - a condition in which a person's body mass exceeds the norm by a certain amount, as measured by body weight index (BMI). A BMI of 30 or greater is considered obese (normal BMI is less than 25).
Obsessive-compulsive disorder (OCD) - an anxiety disorder in which a person has an unreasonable thought, fear, or worry that he/she tries to manage through a ritualized activity to reduce the anxiety. Frequently occurring disturbing thoughts or images are called obsessions, and the rituals performed to try to prevent or dispel them are called compulsions.
Obstetrician/gynecologist (OB/GYN) - physician who specializes in general women's medical care, diagnosis and treatment of disorders of the female reproductive system, and care of pregnant women.
Obstruction - a blockage that prevents the flow of liquids or solids.
Occluded artery - an artery that is narrowed by plaque that impedes blood flow.
Occult - disease or symptoms that are not readily detectable by physical examination or laboratory tests.
Occult bleeding - blood in stool that is not visible to the naked eye.
Ocular hypertension - a condition in which the pressure inside the eyeball is greater than 21 millimeters of mercury (mm Hg), and there is no indication of glaucoma or other cause for the increased pressure.
Odorant - substance that stimulates the sense of smell.
Olfaction - the act of smelling.
Olfactometer - device for estimating the intensity of the sense of smell.
Oligomenorrhea - infrequent or light menstrual cycles.
Oncogenes - a type of mutated gene that causes cells to grow out of control, which can lead to develop of cancer.
"-oma" - a suffix meaning "tumor" or "lump."
Oncologist - a physician who specializes in treating cancer, including subspecialties such as surgical oncologist, radiation oncologist, pediatric oncologist, gynecologic oncologist, and medical oncologist.
On-off effect, on-off phenomena - a change in the patient's condition, with sometimes rapid fluctuations between uncontrolled movements and normal movement, usually occurring after long-term use of levodopa and probably caused by changes in the ability to respond to this drug.
Oophorectomy - surgical removal of one or both ovaries.
Oophoritis - inflammation of the ovary.
Open heart surgery - surgery that involves opening the chest and heart while a heart-lung machine performs for the heart and lungs.
Open surgery - cutting the skin and tissues during surgery to expose a full view of the structures and organs involved in the procedure.
Open-set speech recognition - understanding speech without visual clues.
Ophthalmoscopy - examination of the internal structures of the eye.
Optic nerve - a bundle of more than one million nerve fibers that connects the retina with the brain. The optic nerve is responsible for interpreting the impulses it receives into images.
Optional surgery (also called elective surgery) - an operation the patient chooses to have done, which may not be essential to continuation or quality of life.
Oral and maxillofacial surgeon - a dentist who is responsible for treating a wide variety of dental problems, including the removal of impacted teeth and reconstructive facial surgery. Oral and maxillofacial surgeons complete several more years of training after finishing dental school.
Oral cancer - a cancer found in the oral cavity (mouth), on the lip, and the oropharynx (the part of the throat at the back of the mouth).
Oral dissolution therapy - method of dissolving cholesterol gallstones by taking oral medication.
Orchiectomy (also called castration) - the surgical removal of the testicles.
Orchitis - inflammation of the testicle.
Oropharynx - the part of the throat at the back of the mouth.
Orthodontics - the dental specialty that focuses on the development, prevention, and correction of irregularities of the teeth, bite, and jaws.
Orthokeratology - the use of specialized contact lenses to change the shape of the cornea in order to correct refractive error.
Orthopedic surgeon (also called an orthopedist) - a physician who diagnoses, treats, manages the rehabilitation process, and provides prevention protocols for patients who suffer from injury or disease in any of the components of the musculoskeletal system.
Orthopedic surgery (also called orthopedics) - the medical specialty devoted to the diagnosis, treatment, rehabilitation, and prevention of injuries and diseases of the body's musculoskeletal system.
Orthosis - a brace or splint used to strengthen or stabilize part of the body, usually an arm or leg.
Orthostatic hypotension - a large decrease in blood pressure upon standing; may result in fainting.
Osteitis pubis - an inflammation of the pubic symphysis, the bone to which the two hip bones connect in front of the body.
Osteoarthritis - a condition caused by wear and tear that causes inflammation of the joint, causing swelling, pain, and stiffness.
Osteoblast - a cell that makes bone tissue.
Osteoclast - very large cell formed in bone marrow; its function is to absorb and remove gone tissue.
Osteocyte - cell found within the bone; its function is to help maintain bone as living tissue.
Osteophyte - outgrowth of bone (bone spur).
Osteoporosis - disorder in which bones thin and become brittle and more prone to fracture; most common in women after menopause due to estrogen deficiency.
Ostomy - operation that makes an artificial opening in the abdomen to allow for the release of urine or feces. Colostomy and ileostomy are types of ostomy.
Otitis externa - inflammation of the outer part of the ear extending to the auditory canal.
Otitis media - inflammation of the middle ear caused by infection.
Otoacoustic emissions - low-intensity sounds produced by the inner ear that can be quickly measured with a sensitive microphone placed in the ear canal.
Otolaryngologist - physician/surgeon who specializes in diseases of the ears, nose, throat, head, and neck.
Otologist - physician/surgeon who specializes in diseases of the ear.
Otoplasty (also called ear surgery) - a type of cosmetic plastic surgery procedure aimed at setting prominent ears closer to the head, or reducing the size of larger ears.
Otosclerosis - abnormal growth of bone in the inner ear, which prevents structures within the ear from working properly, resulting in a gradual loss of hearing.
Otoscope - a lighted instrument that allows the physician to see inside the ear.
Ototoxic drugs - drugs that can damage the hearing and balance organs located in the inner ear.
Outer ear - external portion of the ear, consisting of the pinna, or auricle, and the ear canal.
Outpatient surgery - surgery which allows the patient to go home the same day the procedure is done.
Ovaries - pair of small glands, located on either side of the uterus, in which egg cells develop and are stored and the female sex hormones estrogen and progesterone are produced.
Overflow incontinence - leakage of urine that occurs when the quantity of urine produced exceeds the bladder's capacity to hold it.
Overuse conditions - injuries due to minor trauma involving soft-tissue injuries - injuries that affect the bone, muscles, ligaments, and/or tendons.
Ovulation - release of a mature egg from an ovary.
Ovum - a mature egg cell released during ovulation from an ovary.
Oxytocin - hormone produced by the pituitary gland that stimulates contractions of the uterus during labor and release of milk during breastfeeding.