Oligodendrocyte A type of glial cell in the CNS that wraps itself around an axon to form myelin, which serves like insulation. Oncogene Altered or misdirected proto-oncogene which then has the ability to transform the normal cell into a neoplastic cell.  Most oncogenes differ from their proto-oncogenes by a single point mutation. Organ System A group of organs that contribute to specific functions within the body. Organelle An intracellular structure that performs a specific function or group of functions. Organic Compound A substance that contains covalently-bonded carbon and hydrogen and often other elements. Organophosphate Chemical Organic chemicals that contain a phosphate group.  Many are highly toxic, as they are capable of inhibition of the enzyme acetylcholinesterase at neural synapses.  Many pesticides and some warfare agents are organophosphate chemicals. Organs A group of tissues precisely arranged so that so they can work together to perform specific functions.