The oxidation number of the manganese ion in the compound with the chemical formula MnO2 is +4. This compound, called manganese dioxide, also contains two oxygen ions, each with an oxidation number of -2.
As the guidelines for nonmetals indicate, oxygen always has an oxidation number of -2 when paired with metals such as manganese. This requires manganese to take a charge of +4, so the MnO2 compound has a neutral total charge.
The manganese ion also exhibits other oxidation numbers, such as +2 and +7. For example, in KMnO4, the potassium ion has a charge of +1 and each of the oxygen ions has a charge of -2, which causes the manganese ion to take a charge of +7.