Here's why:

* Physical Properties: Oxygen is a gas at room temperature, lacks metallic luster, is a poor conductor of heat and electricity. These are all characteristic of nonmetals.

* Chemical Properties: Oxygen is highly reactive, readily forming compounds with other elements. This reactivity is another key feature of nonmetals.

* Location on the Periodic Table: Oxygen is located in Group 16 (VIA) of the periodic table, which is the group that contains the nonmetals.

While oxygen is essential for life and supports combustion, it is fundamentally a nonmetal based on its physical and chemical properties.