How can gaining oxygen indicate oxidation?

In the context of oxidation-reduction reactions, gaining oxygen is a classic indicator of oxidation because it corresponds to an increase in an atom's oxidation number.1

While modern chemistry defines oxidation broadly as the loss of electrons, the oxygen-atom-transfer criterion remains a fundamental way to identify these processes. When a chemical species gains an oxygen atom, it typically undergoes an increase in its oxidation state because oxygen is highly electronegative, meaning it tends to attract and hold bonding electrons.123

In this transfer, the oxygen atom effectively pulls electron density away from the species it bonds with. Consequently, the atom that gains the oxygen experiences a formal increase in its oxidation number, which is the defining characteristic of oxidation. This principle is consistent across many common processes, such as the formation of metal oxides or the conversion of aldehydes to carboxylic acids.4125