oceanography - the study of the ocean and ocean life.

oil - a black, sticky substance used to produce fuel (petroleum) and materials (plastics).

oil spills - the harmful release of oil into the environment, usually in the water, sometimes killing area flora and fauna. Oil spills are very difficult to clean up.

old growth forests - see ancient forests.

omnibus spending bill - a bill combining the appropriations for several federal agencies.

over-development - expansion or development of land to the point of damage.

over-fishing - fishing beyond the capacity of a population to replace itself through natural reproduction.

over-grazing - grazing livestock to the point of damage to the land.

ozone - a naturally occurring, highly reactive gas comprising triatomic oxygen formed by recombination of oxygen in the presence of ultraviolet radiation. This naturally occurring gas builds up in the lower atmosphere as smog pollution, while in the upper atmosphere it forms a protective layer which shields the earth and its inhabitants from excessive exposure to damaging ultraviolet radiation.

ozone depletion - the reduction of the protective layer of ozone in the upper atmosphere by chemical pollution.

ozone hole - a hole or gap in the protective layer of ozone in the upper atmosphere.