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Odorants & Deodorants: Odorants are used to add odor to materials, usually for safety reasons.

Opaque: Not able to transmit light.

Orange Peel: A surface finish on a molded part that is rough and splotchy. Usually caused by moisture in the mold cavity or poor heat transfer properties.

Orientation: the arrangement of the molecules of the melt. If the molecules are orientated, they are aligned with each other; if non-orientated they are not in alignment. In general, the orientated material shrinks more than non-orientated material.

Orifice: opening.

Over Molding: A process in which a mold cavity is first partially filled with one plastic and then the second shot is injected to encapsulate the first shot.

Overpack: melt will fill the easiest flow path first and will continue to pack this area while material reaches the other areas. This is a cause of warping created by the unbalanced flow.

Oxygen Index: A flammability test based on the principle that a certain volumetric concentration of oxygen is necessary to maintain combustion of a specimen after it has been ignited.