The oxidation state, often called the oxidation number,(and OS or ON for short) is an indicator of the degree of oxidation (loss of electrons) of an atom in a chemical compound. Conceptually, the oxidation state, which may be positive, negative or zero, is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic, with nocovalent component. This is never exactly true for real bonds. Oxidation states are typically represented by small integers. In some cases, the average oxidation state of an element is a fraction, such as 8/3 for iron in magnetite (Fe3O4).

The increase in oxidation state of an atom through a chemical reaction is known as an oxidation; a decrease in oxidation state is known as a reduction. Such reactions involve the formal transfer of electrons, a net gain in electrons being a reduction and a net loss of electrons being an oxidation.

For pure elements, the oxidation state is zero i.e. all the elements in their gaseous phase have zero OS.