Oxygen is a highly reactive and very abundant element that reacts with many things, including, as the University of California, Davis' ChemWiki points out, the reaction it has with hydrogen, which creates water. This element is so reactive that it can even react with itself.
Oxygen reacts with several main group elements, including lithium, sodium, potassium, magnesium, calcium, barium, boron, aluminum, nitrogen, phosphorus, sulfur, fluorine and xenon. Chemical reactions do not necessarily have to be dramatic events, and many of oxygen's reactions with other elements are relatively mundane. Because it is both highly reactive and very naturally abundant, oxygen is commonly found in elemental compounds such as mercuric oxide (HgO).