Obesity: Overweight. Please see MedicineNet site on OBESITY for more information.

OB/GYN: A commonly used abbreviation. GYN is short for gynecology (or a gynecologist).

Obsessive-compulsive: A form of personality (or personality disorder) marked by obsessions and compulsions.

Obstetrician: A physician who delivers babies and is in the practice of obstetrics.

Obstetrics: The art and science of managing pregnancy, labor and the pueperium (the time after delivery).

Obtunded: Mentally dulled. Head trauma may obtund a person.

Occipital bone: The bone forming the rear (and rear bottom) of the skull.

Occiput: The back of the head.

Occular: Having to do with the eye.

Occult: Hidden. Occult blood is hidden from the eye but is nontheless present and can be detected by chemical tests.

OCP: Oral contraceptive pill. Also known as "the pill".

Ocular: Having to do with the eye.

Oesophagus: Alternate spelling for esophagus.

Ointment: An ointment has an oil base whereas a cream is water-soluble. (The word ointment comes from the Latin ungere meaning anoint with oil).

Olfaction: The sense of smell. Olfactory apparatus: The whole system needed to have a sense of smell.

Oligo-: Means just a few or scanty.

Oligodendroglioma: A type of brain tumor.

Oligohydramnios: Scant amniotic fluid: less than usual.

Oligomenorrhea: Scant menstruation. Less menstrual blood flow than usual.

Oligospermia: Fewer sperm than usual. Azospermia, by contrast, means absolutely no sperm at all.

Oliguria: Less urination than normal.

Oligonucleotide: A short DNA molecule composed of relatively few nucleotide bases.

Omentum: A sheet of fat covered by peritoneum. The greater omentum is attached to the bottom edge of the stomach and hangs down in front of the intestines. The other edge is attached to the transverse colon. The lesser omentum is attached to the top edge of the stomach and extends to the undersurface of the liver.

Colon - add the following to the end of the existing text: The colon can be divided into ascending (right), transverse, descending (left), and sigmoid segments. The ascending colon includes the cecum which joins with the small bowel and also has the appendix attached to it. Stool then proceeds up the ascending colon toward the liver, then across to the left side of the body via the transverse colon. Near the spleen, the colon turns downward and becomes the descending colon which becomes the sigmoid colon toward the pelvis.

Ommaya reservoir: A device implanted under the scalp and used to deliver anticancer drugs to the fluid surrounding the brain and spinal cord.

Omphalocele: A birth defect with intestine protruding outside the abdomen at the umbilicus. Due to a failure at 10 weeks' embryonic development for the intestine, for a time normally outside the abdomen, to return.

Oncogene: A gene that plays a normal role in cell growth and, when altered, may contribute to the growth of a tumor. The word oncogene (literally, cancer gene) is catchy but oversimplifies the complex process of cancer.

Oncologist: A doctor who specializes in treating cancer.

Onycho-: Having to do with the nails.

Onychodystrophy: Malformation of the nails. Oo-: Prefix meaning egg. Pronounced o'-o.

Onychoosteodysplasia: The nail-patella syndrome, an hereditary condition with abnormally formed (dysplastic) or absent nails and absent or underdeveloped (hypoplastic) kneecaps (patellae). Other features include iliac horns, abnormality of the elbows interfering with full range of motion (pronation and supination) and kidney disease resembling glomerulonephritis which.is often mild but can be progressive and lead to renal failure. Onychoosteodysplasia is inherited as dominant gene. This means that the disease can be transmitted by one affected parent. The nail-patella gene locus found linked genetically to the ABO blood group in1965 is now known to be in chromosome region 9q34. Also called Turner-Kieser syndrome, and Fong disease.

Oocyte: A female germ cell in the works; a developing egg cell.

Oogonium: Ancestral cell that gives rise to oocytes.

Oophorectomy: The removal of one or both ovaries. Oophoritis: Inflammation of the ovary.

Open reading frame: (genetics) An open reading frame in DNA has no termination codon, no signal to stop reading the nucleotide sequence, and so may be translated into protein.

Ophthalmoscope: A lighted instrument used to examine the inside of the eye, including the retina and the optic nerve.

Ophthalmia: Inflammation of the eye.

Ophthalmic: Pertaining to the eye. An ophthalmic ointment is designed for the eye.

Ophthalmologist: An eye doctor. A physician practicing ophthalmology. An ophthalmologist is an M.D.

Ophthalmology: The art and science of eye medicine.

Ophthalmopathy: Eye disease.

Optic: Having to do with vision.

Optic neuroma: A benign tumor of the optic nerve.

Optic nerve: The nerve that carries Messages from the retina to the brain.

Optometrist: Professional trained to provide primary eye and vision care and improve vision with glasses, contact lenses, etc. An optometrist is an O.D. (Doctor of Optometry), not an M.D.

Oral: Having to do with the mouth.

Oral cancer: Cancer within the mouth. Please see MedicineNet site on RAL CANCER for more information.

Oral surgeon: A dentist with special training in surgery of the mouth and jaw.

Orbit: In medicine, the bony cavity in which the eyeball sits.

Orbital ridge: The bony ridge beneath the eyebrow.

Orchitis: Inflammation of the testis (male sex organ). There are many causes of inflammation of the testis including infections (such as mumps), diseases (such as polyarteritis nodosa), or injury. Also called orchiditis.

Orchiectomy: The surgical removal of the testicles.

Orifice: An opening. The mouth for example is an orifice.

Oromandibular dystonia: Oro- refers to the mouth and mandibular refers to the lower jaw. Oromandibular dystonia affects the muscles of the jaw, lips, and tongue. The jaw may be pulled either open or shut, and speech and swallowing can be difficult.

Oropharynx: The area of the throat at the back of the mouth.

Ortho-: Prefix meaning straight or erect.

Orthodontic treatment (dental braces): Please see MedicineNet site on ORTHODONTIC TREATMENT for information on this subject.

Orthopedics: Literally, the practice of child straightening, orthopedics is the branch of surgery broadly concerned with the skeletal system (bones).

Orthopedist: An orthopedic surgeon.

Orthopod: Slang for orthopedist.

Orthopnea: The plight of a person who can only breath easily when sitting straight or standing erect.

Orthostatic hypotension: A temporary low blood pressure (hypotension) due usually to suddenly standing up (orthostatic). Symptoms such as dizziness, feeling about to black out, and tunnel vision can be due to insufficient blood flow to the brain. The symptoms are typically worse when standing and improve with lying down. Orthostatic hypotension may be experienced by healthy people who rise quickly from a chair, especially after a meal, and have a few seconds of disorientation.

Osseous: Having to do with the bone.

Osteo-: Prefix meaning bone.

Osteoarthritis: Type of arthritis caused by inflammation, breakdown and eventual loss of the cartilage of the joints. For more information on this common type of arthritis, please see the MedicineNet site on OSTEOARTHRITIS.

Osteochondroma: An abnormal solitary benign growth of bone and cartilage typically at the end of a long bone. Osteochondromas are usually discovered in persons 15 to 25 years of age. They are typically detected when injured or they become large enough to be noted.

Osteodystrophy (renal osteodystrophy): A combination of bone disorders usually caused by chronic kidney failure (renal disease). Please see the MedicineNet site on OSTEODYTROPHY for more information.

Osteogenesis: The production of bone.

Osteogenesis imperfecta: Brittle bone disease.

Osteomalacia: Softening of the bone.

Osteomyelitis: Inflammation of the bone due to infection, for example by the bacteria salmonella.

Osteonecrosis (aseptic necrosis or avascular necrosis): Condition resulting from poor blood supply to an area of bone causing bone death. For more information on this condition, please see the MedicineNet site on OSTEONECROSIS.

Osteopath: An osteopathic physician who is a Doctor of Osteopathy (D.O.).

Osteopathy: A system of therapy founded in the 19th century based on the concept that the body can formulate its own remedies against diseases when the body is in a normal structural relationship, has a normal environment and enjoys good nutrition.

Osteopetrosis: Stony bones; thickening of the bones, also called marble bones.

Osteoporosis: Thinning of the bones with reduction in bone mass due to depletion of calcium and bone protein, predisposing to fractures. For more information, please see the MedicineNet site on OSTEOPOROSIS.

Osteosarcoma: A cancer of the bone that is most common in children. Also called osteogenic sarcoma.

Osteotomy: Taking out part or all of a bone.

Ostomy: An operation to create an opening from an area inside the body to the outside. See glossary entry for colostomy.

Otitis: Inflammation of the ear.

Otitis externa: Inflammation of the external ear canal that leads inward to the ear drum (tympanic membrane).

Otitis interna: Inflammation of the inner ear.

Otitis media: Inflammation of the middle ear, often due to an infection. Very common in children.

Oto-: Prefix meaning ear.

Otolaryngologist: A doctor who specializes in treating diseases of the ear, nose, and throat.

Otoscope: Instrument for looking in the ear. Today, otoscopic or ophthalmoscopic heads can usually be attached to the base (which supplies the electrical power) to look at the ears or eyes.

Ounce: A measure of weight equal to 1/16th of a pound or, metrically, 28.35 grams. The abbreviation for ounce is oz. (An ounce of prevention is reputedly worth a pound of cure.)

Outpatient: A patient who is not an inpatient (not hospitalized). Outpatient care is called ambulatory care.

Ova: Two or more "ovums". (Ova is the plural of ovum).

Ovarian cancer: Cancer of the ovary. Most often due to ovarian carcinoma. For more information on this important women's health problem, please see the MedicineNet site on OVARIAN CANCER.

Ovaries: The pair of female reproductive organs that produce eggs (ova) and hormones. They are located in the lower abdomen, one on each side of the uterus.

Ovary: An ovary or "egg sac" is one of a pair of reproductive glands in women. The ovaries produce the ovum or female egg which is transferred through the fallopian tube for fertiliztion by the sperm. The ovary also produces both the hormones estrogen and progesterone.

Overgrowth: Just what it sounds like: excessive growth.

Overgrowth syndromes: Conditions with multiple abnormalities including excessive growth. Early overgrowth syndromes that affecting children include the fragile X and Beckwith-Wiedemannn syndromes. Overactivity of the pituitary gland with overproductiuon of growth hormone causes overgrowth before adolescence and a distinctive pattern of overgrowth called acromegaly.

Overweight: Obesity. Please see MedicineNet site on OBESITY for information.

Ovulation: The release of the ripe egg from the ovary.

Ovum: An ovum is an egg that exists in the ovary of the female. This egg is called the female "gamete" or sex cell. It combines with the male gamete, called a sperm, to form a zygote. This formation process is called "fertilization." (see sperm, zygote).

Oxyuris: A group of intestinal worms that includes pinworm.

oz.: Abbreviation for ounce.