Oil Desulphurisation
Pre-combustion method for reducing sulphur dioxide emissions from oil-burning power plants. The oil is treated with hydrogen, which removes some of the sulphur by forming hydrogen sulphide gas.

Oil Spill
An accidental or intentional discharge of oil, which reaches bodies of water. Can be controlled by chemical dispersion, combustion, mechanical containment, and/or adsorption. Spills from tanks and pipelines can also occur away from water bodies, contaminating the soil, getting into sewer systems and threatening underground water sources.

Opacity
The amount of light obscured by particulate pollution in the air.

Optimisation
Working in the most efficient or most desirable mode of operation.

Oral
Spoken or communicated by mouth.

Organic Chemicals/Compounds
Animal or plant-produced substances containing mainly carbon, hydrogen, nitrogen, and oxygen.

Organic Matter
Carbonaceous waste contained in plant or animal matter and originating from domestic or industrial sources.

Organic
1. Referring to or derived from living organisms. 2. In chemistry, any compound containing carbon.

Organism
Any form of animal or plant life.

Organophosphates
Pesticides that contain phosphorus; short-lived, but some can be toxic when first applied.

Organotins
Chemical compounds used in anti-foulant paints to protect the hulls of boats and ships, buoys, and pilings from marine organisms such as barnacles.

Osmosis
The passage of a liquid from a weak solution to a more concentrate solution across a semi-permeable membrane that allows passage of the solvent (water) but not the dissolved solids.

Outfall
The place where effluent is discharged into receiving waters.

Ozone (O3)
Found in two layers of the atmosphere, the stratosphere and the troposphere. In the stratosphere (the atmospheric layer 7 to 10 miles or more above the earths surface) ozone is a natural form of oxygen that provides a protective layer shielding the earth from ultraviolet radiation. In the troposphere (the layer extending up 7 to 10 miles from the earth's surface), ozone is a chemical oxidant and major component of photochemical smog. It can seriously impair the respiratory system. Ozone in the troposphere is produced through complex chemical reactions of nitrogen oxides, which are among the primary pollutants emitted by combustion sources; hydrocarbons, released into the atmosphere through the combustion, handling and processing of petroleum products; and sunlight.

Ozone depletion
Destruction of the stratospheric ozone layer, which shields the earth from ultraviolet radiation harmful to life. This destruction of ozone is caused by the breakdown of certain chlorine and/or bromine containing compounds (chlorofluorocarbons or halons), which break down when they reach the stratosphere and then catalytically destroy ozone molecules.

Ozone Hole
Thinning break in the stratospheric ozone layer. Designation of amount of such depletion as a "ozone hole" is made when detected amount of depletion exceeds fifty percent. Seasonal ozone holes have been observed over both the Antarctic region and the Arctic region and part of Canada and the extreme north-eastern United States.

Ozone Layer
The protective layer in the atmosphere, about 15 miles above the ground, that absorbs some of the sun's ultraviolet rays, thereby reducing the amount of potentially harmful radiation reaching the earth's surface.