Off-ratio mix—When the mixture of isocyanate and resin does not conform to the manufacturer's recommended mixing ratio. The acceptable ratio for most systems is when the two components are combined in equal volumes.

Olefin—An unsaturated open-chain hydrocarbon containing at least one double bond: ethylene or propylene.

Olefin plastics—Plastics based on polymers made by the polymerization of olefms or copolymerization of olefins with other monomers, the olefms being at least 50 mass %.

One-on-one—The application of a single ply of roofing over the substrate, followed by the application of a second single ply over the first (phased application).

Open web joist—(See Bar joists).

Operational amplifier—A device used in electronic instrumentation to amplify electrical signals.

Organic, adj.—Being or composed of hydrocarbons or their derivatives, or matter of plant or animal origin.

Organic coating—Coatings that are generally inert or inhibited. May be temporary (e.g., slushing oils) or permanent (paints, varnishes, enamels, etc.).

Organic content—Usually synonymous with volatile solids in an ashing test; e.g., a discrepancy between volatile solids and organic content can be caused by small traces of some inorganic materials such as calcium carbonate that lose weight at temperatures used in determining volatile solids.

Osmosis—The diffusion of fluids through a semipermeable membrane or porous partition.