O(6)-benzylguanine: A drug that may improve the response of cancer cells to chemotherapy.

oat cell cancer: A type of lung cancer in which the cells look like oats when viewed under a microscope. Also called small cell lung cancer.

observation: Closely monitoring a patient's condition but withholding treatment until symptoms appear or change. Also called watchful waiting.

obstruction: Blockage of a passageway.

obtundation (ob-tun-DAY-shun): A dulled or reduced level of alertness or consciousness.

octreotide: A drug similar to the naturally occurring growth hormone inhibitor somatostatin. Octreotide is used to treat diarrhea and flushing associated with certain types of tumors.

ofloxacin: An antibiotic drug used to treat infection. It belongs to the family of drugs called quinolone antibiotics.

oligodendroglial tumors: Rare, slow-growing tumors that begin in brain cells called oligodendrocytes, which provide support and nourishment for cells that transmit nerve impulses. Also called oligodendroglioma.

oligodendroglioma (OL-ih-go-den-dro-glee-O-ma): A rare, slow-growing tumor that begins in brain cells called oligodendrocytes, which provide support and nourishment for cells that transmit nerve impulses. Also called oligodendroglial tumor.

oltipraz: A drug used in cancer prevention.

omega-3 fatty acid: A type of fat obtained in the diet and involved in immunity.

omentum (oh-MEN-tum): A fold of the peritoneum (the thin tissue that lines the abdomen) that surrounds the stomach and other organs in the abdomen.

omeprazole: A drug that inhibits gastric acid secretion.

Ommaya reservoir (o-MYE-a REZ-er-vwahr): A device surgically placed under the scalp and used to deliver anticancer drugs to the fluid surrounding the brain and spinal cord.

oncogene: A gene that normally directs cell growth. If altered, an oncogene can promote or allow the uncontrolled growth of cancer. Alterations can be inherited or caused by an environmental exposure to carcinogens.

oncologist (on-KOL-o-jist): A doctor who specializes in treating cancer. Some oncologists specialize in a particular type of cancer treatment. For example, a radiation oncologist specializes in treating cancer with radiation.

oncology: The study of cancer.

oncology nurse: A nurse who specializes in treating and caring for people who have cancer.

ondansetron: A drug that prevents or reduces nausea and vomiting.

ONYX-015: A modified cold virus that selectively grows in and destroys certain types of cancer cells and leaves normal cells undamaged.

oophorectomy (o-o-for-EK-toe-mee): Surgery to remove one or both ovaries.

ophthalmoscope (off-THAL-mo-skope): A lighted instrument used to examine the inside of the eye, including the retina and the optic nerve.

optic nerve: The nerve that carries messages from the retina to the brain.

oral: By or having to do with the mouth.

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

orchiectomy (or-kee-EK-toe-mee): Surgery to remove one or both testicles.

organism: A living thing, such as an animal, a plant, a bacterium, or a fungus.

oropharynx (or-o-FAIR-inks): The middle part of the throat that includes the soft palate, the base of the tongue, and the tonsils.

osteogenic sarcoma: A malignant tumor of the bone. Also called osteosarcoma.

osteolytic: Causing the breakdown of bone.

osteoporosis (OSS-tee-oh-pa-ROW-sis): A condition that is characterized by a decrease in bone mass and density, causing bones to become fragile.

osteosarcoma (AHS-tee-o-sar-KO-ma): A cancer of the bone that affects primarily children and adolescents. Also called osteogenic sarcoma.

ostomy (AHS-toe-mee): An operation to create an opening (a stoma) from an area inside the body to the outside. Colostomy and urostomy are types of ostomies.

otolaryngologist (AH-toe-lar-in-GOL-o-jist): A doctor who specializes in treating diseases of the ear, nose, and throat. Also called an ENT doctor.

ovarian: Having to do with the ovaries, the female reproductive glands in which the ova, or eggs, are formed. The ovaries are located in the pelvis, one on each side of the uterus.

ovarian ablation: Surgery, radiation therapy, or a drug treatment to stop the functioning of the ovaries. Also called ovarian suppression.

ovarian epithelial cancer: Cancer that occurs in the cells lining the ovaries.

ovaries (O-va-reez): The pair of female reproductive glands in which the ova, or eggs, are formed. The ovaries are located in the pelvis, one on each side of the uterus.

overall survival: The percentage of subjects in a study who have survived for a defined period of time. Usually reported as time since diagnosis or treatment. Often called the survival rate.

overexpress: An excess of a particular protein on the surface of a cell.

ovulation (ov-yoo-LA-shun): The release of an egg from an ovary during the menstrual cycle.

oxaliplatin: An anticancer drug that belongs to the family of drugs called platinum compounds.

OXi-104: An anticancer drug being evaluated in combination with cisplatin.

oxidative metabolism: A chemical process in which oxygen is used to make energy from carbohydrates (sugars). Also known as aerobic respiration, cell respiration, or aerobic metabolism.

oxidative stress: A condition in which antioxidant levels are lower than normal. Antioxidant levels are usually measured in blood plasma.