Here's why:

* Pure substance: A pure substance is made up of only one type of particle. Oxygen gas is composed solely of oxygen molecules (O₂).

* Mixture: A mixture is a combination of two or more substances that are not chemically bonded.

* Homogeneous mixture: A homogeneous mixture has a uniform composition throughout.

Since oxygen gas is made up of only one type of particle, it's not a mixture. It's a pure substance.