Oxygen is classified as a pure substance. This classification is due to the fact that oxygen, in its most common form (O2), consists of only one type of molecule. A pure substance is defined as a material that contains only one type of particle, and that has a consistent and uniform composition throughout.
To elaborate, a mixture contains two or more different substances that can be separated by physical means. Examples of mixtures include air (which contains nitrogen, oxygen, carbon dioxide, and other gases) and saltwater (which combines salt and water but retains the properties of both). In contrast, pure oxygen does not vary in composition; it is always made up entirely of diatomic oxygen molecules. Because of its consistent chemical structure and properties, such as its behavior during combustion and its role in respiration, oxygen qualifies as a pure substance.