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: A result of an experiment or trial in which a variable, either numerical or categorical, is measured.

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. Octreostatin. 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 type of brain 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 abdome 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. Sting 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.

Ovaries: Surgery to remove one or both ovaries.

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 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.