Oligodendrocyte: Specialized cell surrounding the nerve cells of the central nervous system (CNS) in vertebrates whose function is to myelinate CNS axons, the long process of nerve cell that carry outgoing information. Omega-3 fatty acids: A class of fatty acids that have a double bond three carbons from the methyl moiety; reportedly, they play a role in lowering cholesterol and LDL levels. Oncogene: Gene, expression (production of protein) of which causes cancer. These genes were originally found in cancer causing viruses where their presence was shown to be critical for producing cancer. Oncologists: Physicians specially trained to treat cancer patients. They may specialize in various branches of the treatment, e.g., chemotherapy, surgery or radiotherapy. Oncoprotein: Protein product of oncogene. Oophorectomy: Removal of ovary. Opioid: A synthetic narcotic that resembles the naturally occurring opiates and binds to the opiate receptor on cell surface eliciting its effect. Osmolarity: The concentration of osmotically active particles in solutions expressed in terms of osmoles of solute per liter of solution. Osmotically balanced: Flow of pure solvent from a solution of lesser to one of greater solute concentration is generated, when the two solutions are separated by a membrane which selectively prevents the passage of the solute molecules, but is permeable to the solvent. This flow can cause swelling and subsequent rupture of the cells. If the total nonpermeable solutes are the same on both sides of the membrane it is osmotically balanced and rupture is prevented. Osteoclast: A large multinuclear cell associated with the absorption and removal of bone. Osteoclasts become highly active in the presence of parathyroid hormone. Osteomalacia: A condition marked by softening of the bones (due to impaired mineralisation, with excess accumulation of osteoid), with pain, tenderness, muscular weakness, anorexia and loss of weight, resulting from deficiency of vitamin D and calcium. Ovarian follicle: In mammals the groups of cells around the primary oocyte proliferate and form a surrounding noncellular layer. Ovaries: Reproductive organs in female containing ova situated in the lower abdomen on each side of the uterus. Ovaries produce hormones including estrogen and progesterone. Ovulation: The discharge of a secondary oocyte from the follicle of the ovary. Oxidation: The act of oxidizing or state of being oxidized. Chemically it consists in the increase in the positive charges in an atom or loss of negative charges. Most biological oxidation is accompanied by the removal of a pair of hydrogen atoms (dehydrogenation) from a molecule. Such oxidation must be accompanied by reduction (acquisition of hydrogen atom) of an acceptor molecule. Oxidative stress: A highly oxidized environment within cells because cells are forced into a highly activated state due to loss of control of their regulatory systems. Oxidative damage: Damage produced by the process of oxidation usually due to the formation of active radicals. Oxygen radical: A species of oxygen extremely reactive and having a very short life.