The formula for an oxide ion is O2-, which shows that it is an anion. An oxide is a compound formed by the combination of oxygen and another element. Oxygen can form many different oxide compounds with both metals and nonmetals.
Metal oxides can be compounds that contain alkali metals, alkaline earth metals and transition metals. In these compounds, the metal is the cation, and the oxide is the anion. When metal oxides react with water, they produce a basic solution. The reaction between nonmetals and water produces an acidic solution.
On the periodic table, some common trends exhibited by oxides are that down a group the basicity of oxides tend to increase, and across the periods their bonding changes from ionic to covalent.