obstruction - blockage in the gastrointestinal tract 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 - high (greater than 21 mm Hg) intraocular 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.

oncogenes - genes that promote normal cell division.

oncologist - a physician who specializes in treating cancer, including surgical oncologist, radiation oncologist, pediatric oncologist, gynecologic oncologist, and medical oncologist. 

oophorectomy - surgery to remove one or both ovaries.

open surgery - cutting the skin and tissues during surgery to expose a full view of the structures and organs involved in the procedure.

optional surgery - an operation the patient chooses to have done, which may not be essential to the continuation or quality of life. (See also elective surgery.)

oral and maxillofacial surgeon - orthopaedic facial surgeon who is responsible for treating a wide variety of dental problems, including the removal of impacted teeth and reconstructive facial surgery.

oral dissolution therapy - method of dissolving cholesterol gallstones.

orchiectomy (Also called castration.) - the surgical removal of the testicles.

oropharynx - the part of the throat at the back of the mouth.

orthodontics - orthodontics is the dental specialty that focuses on the development, prevention, and correction of irregularities of the teeth, bite, and jaw.

orthokeratology - the use of contact lenses to change the shape of the cornea in order to correct refractive error.

orthopaedic surgery (Also called orthopaedics.) - the medical specialty devoted to the diagnosis, treatment, rehabilitation, and prevention of injuries and diseases of the body's musculoskeletal system.

orthostatic hypotension - a large decrease in blood pressure upon standing; may result in fainting.    

osteoblast - cell found in bone; its function is to form the tissue and minerals that give bone its strength. 

osteoclast - very large cell formed in bone marrow; its function is to absorb and remove unwanted tissue.

osteocyte - cell found within the bone; its function is to help maintain bone as living tissue.

osteophyte - an outgrowth of bone.

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.

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.  

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.

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 - the leakage of small amounts of urine from a bladder that is always full.

ovulation - release of a mature egg from an ovary.

oxytocin - hormone produced by the pituitary gland that stimulates contractions of the uterus during labor and release of milk during breastfeeding.