In coordination chemistry, the oxidation number of a central atom in a coordination compound is the charge that it would have if all the ligands were removed along with the electron pairs that were shared with the central atom (in a coordination compound, all shared electron pairs are donated by the ligands). Imagine the central atom of a molecule being stripped of all its appendages, and the intervening electrons going with the ligands, leaving the central atom 'naked.' So, for example, in water, the central atom is oxygen, and the ligands are the two hydrogens which came to the oxygen to share its electrons. In stripping, the hydrogens go away along with the two electrons they came to the oxygen for, leaving the oxygen minus 2 electrons. Thus, oxygen has an oxidation number of +2 (the absence of 2 negative charges, the electrons, is effectively equivalent to the 'presence' of 2 positive charges).

The oxidation number is used in the nomenclature of inorganic compounds. It is represented by a Roman numeral; the plus sign is omitted for positive oxidation numbers. The oxidation number is placed either as a right superscript to the element symbol, e.g. Fe, or in parentheses after the name of the element, e.g. iron(III): in the latter case, there is no space between the element name and the oxidation number.

The oxidation number is usually numerically equal to the oxidation state and so the terms oxidation state and oxidation number are often used interchangeably. To be more precise, however, oxidation number is used in coordination chemistry with a slightly different meaning since the rules used for counting electrons are different: every electron belongs to the ligand, regardless of electronegativity. Also, oxidation numbers are conventionally represented with Roman numerals while oxidation states use Arabic numerals. The oxidation state can differ from the oxidation number in a few cases where the ligand atom is less electronegative than the central atom (e.g., in iridium phosphine complexes), resulting in a formal oxidation state that is different from the oxidation number.