Why is oxygen considered an oxidizing agent in reactions that form oxides?

Oxygen is considered an oxidizing agent because of its high electronegativity and electron affinity, which drive it to accept electrons from other substances during chemical reactions.1

When oxygen reacts with an electron-donating substance, it undergoes a process called reduction, where its own oxidation state is lowered—typically from zero in its molecular form ($O_2$) to $-2$ in the resulting oxide compound. This electron transfer, whether complete or partial, is the defining characteristic of an oxidizing agent.1

Because oxygen frequently acts as the electron acceptor in these reactions, the very nomenclature used to describe oxidation and reduction is based on this typical behavior of oxygen, which is often regarded as the original oxidizing agent. In the formation of oxides, oxygen effectively strips electrons from metals or nonmetals, facilitating the creation of ionic or covalent bonds that define the oxide class.12