Oxygen belongs to group 16 (the chalcogens), has atomic number 8, mass of 16.00 amu, and electron configuration of [He] 2s2 2p4. It was discovered in 1772 by Carl Wilhelm Scheele.
Oxygen is a highly reactive, non-metallic element and forms compounds with most elements with the exception of the inert Noble Gasses (Group 18/8A). It is a strong oxidizing agent (can cause corrosion such as rust), and is fairly electronegative. Oxygen gas O2 makes up about 20.8% of the earth's atmosphere. This gas is produced by photosynthetic organisms, without which O2 would deplete considerably due to it's high reactivity. Ozone, O3, protects the biosphere from harmful UVB radiation.
Aside from Carbon and Hydrogen, Oxygen is arguably one of the most important elements to biology. It is essential to the formation of water, H2O, as well as most organic molecules such as proteins, nucleic acids (DNA/RNA), carbohydrates, and fatty acids. O2 is also necessary for cellular respiration, the way we produce energy (ATP) in the citric acid cycle.