From Envirowiki

Oxygen is an element that is gaseous at room temperature. It makes up about 21% of earth's atmosphere, and most animals breathe it in. It is split from carbon dioxide by plants. Oxygen is poisonous to animals above a certain concentration, however this rarely happens on earth, and more likely is suffocation due to lack of oxygen.

Oxygen is required for the burning of carbon-based organic matter (which is a form of oxidisation). Oxygen also rusts iron and aluminium and a number of other metals (also oxidisation)