Oncogene: A type of gene. When these genes are abnormally "turned on" (activated), they cause excessive growth and other characteristics of malignancy.

Oncologist: A physician who is specially trained in the diagnosis and treatment of cancer. Medical oncologists specialize in the use of chemotherapy and other drugs to treat cancer. Radiation oncologists specialize in the use of x-rays and other radiation to kill tumors. Surgical oncologists specialize in performing operations to remove cancer.

Oncology: The branch of medicine that deals with cancer and tumors.

Oncology nurse specialist: A registered nurse who has taken additional courses and specialized training in the care of cancer patients. Oncology nurse specialists may prepare and administer treatments, monitor patients, prescribe and provide aftercare, and teach and counsel patients and their families. Some oncology nurse specialists are also certified nurse practitioners. (See also case manager, nurse practitioner).

Oncology social worker: A person with a Master's degree in social work who has specialized in working with cancer patients. The oncology social worker provides counseling and assistance to people with cancer and their families, especially in dealing with the non-medical crises that can result from cancer, such as financial problems, housing when treatments must be taken at a facility far away from home, and child care.

One-step procedure: Surgery during which the procedure to diagnose the presence of breast cancer (see biopsy) is followed immediately by treatment (such as mastectomy). The patient is given general anesthesia and does not know until she wakes up if the diagnosis was cancer or if a mastectomy was performed. Once the only option in breast cancer treatment, the one-step procedure is now rarely used. (See also two-step procedure).

Oophorectomy: Surgery to remove the ovaries, the primary source of estrogen. It may be performed to remove a lump, tumor, or abscess, or to treat endometriosis.  Oophorectomy is also a preventive measure to reduce the risk of breast cancer by stopping the production of estrogen.

Osteoporosis: Breakdown of bone, resulting in diminished bone mass and reduced bone strength. Osteoporosis can cause pain, deformity (especially of the spine), and fractures (broken bones). This condition is common among post-menopausal women. (See also estrogen replacement therapy).

Ovarian cancer: A cancer of the ovary.  The ovary is one of the pair of female sexual reproductive organs (gonads) found on each side of the lower abdomen, beside the uterus.  Ovarian cancer occurs in approximately 1 in 8000 women with 90% of cases in women over age 40.  It is usually fairly aggressive.

Ovary: Reproductive organ in the female pelvis. Normally a woman has two ovaries. They contain the eggs (ova) that, when joined with sperm, result in pregnancy. Ovaries are also the primary source of estrogen. (See also estrogen).