Ocean

The great body of water that covers more than 70% of ther earth's surface.

Offspring

The infant of an organism, the result of sexual reproduction.

Oligocene

An epoch of time lasting from 38 million to 26 million years ago.

Order

A unit of scientific classification after classes but before families in terms of biological relationships.

Organism

A living being.

Outcrossing

Breeding or mating of individuals of different strains of the same species.

Overpopulation

The growth of a population to the point where its environment can't sustain it.

Oxidation

A chemical reaction in which a substance loses electrons. Oxidation occurs inside organisms when sugar combines with oxygen to produce carbon dioxide, water, and energy.

Oxide

A chemical compound of oxygen with some other element.

Oxygen

A life-supporting gas and a chemical element. Nearly all living things need oxygen to stay alive.