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Oil length
Ratio of oil to resin expressed as a percentage of oil by weight in the resin. Used to determine the physical properties of a resin.

Opacifier
Extender additive used in decorative and architectural paints to save titanium dioxide pigment and reduce cost.

Opacity
Ability of a coating film to prevent the transmission of light and therefore to obliterate or hide the color of the surface to which it is applied.

Open time
Period of time during which the coating application remains re-brushable. Affected by many factors, including temperature, substrate, binder... .

Optical brightener
Additive imparting pure to bluish brightness to polymers and intensifying the degree of whiteness in ready-to-use white pigmented coatings. Its function is based on the fluorescence effect. It absorbs near-ultraviolet radiant flux and re-emits it as violet-blue visible light causing a whiter appearance, especially when added to a yellowish-white material.

Orange peel
Coating defect: surface bumpiness or waviness that resembles the skin of an orange. Orange peel is often caused by poor leveling and is a common defect in both spray and roll applied coatings. For some coating appliances, an orange peel effect may be desirable.

Osmosis
Diffusion of liquid through a paint film or other such membrane.

Overspray
Sprayed coating that is dry when it hits the surface, resulting in dusty, granular adhering particles. Can cause gloss loss with gloss paints or mottling with flats paints.

Oxidative polymerization
Mechanism of drying unsaturated binders (cooked oils, alkyds etc.) in relatively thin films in the presence of atmospheric oxygen, initiated and catalyzed by driers.