Here's why:

* Location on the Periodic Table: Oxygen is located in Group 16 (also known as the chalcogens) of the periodic table, which primarily consists of nonmetals.

* Physical Properties: Oxygen is a colorless, odorless gas at room temperature. Nonmetals generally have low melting and boiling points, are poor conductors of heat and electricity, and are often brittle.

* Chemical Properties: Oxygen is a highly reactive element that readily forms compounds with other elements. This is a characteristic of nonmetals.

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