What is the chemical formula for oxygen?

The chemical formula for oxygen is represented as O2, which signifies that each molecule of oxygen gas consists of two oxygen atoms bonded together. This diatomic nature of oxygen is crucial for understanding many biological and chemical processes, as it is the form of oxygen that is commonly found in the Earth's atmosphere and is essential for respiration in most multicellular organisms.

In contrast, the other options represent different compounds or forms of oxygen. O3 refers to ozone, which consists of three oxygen atoms and is a different allotrope of oxygen with distinct properties, primarily found in the stratosphere. CO2 is carbon dioxide, composed of one carbon atom and two oxygen atoms; it is a significant greenhouse gas and a product of combustion. H2O represents water, which consists of two hydrogen atoms and one oxygen atom; while it contains oxygen, it is not a representation of elemental oxygen. Thus, O2 is the correct answer as it specifically denotes the diatomic molecule of oxygen that is vital for life and numerous chemical reactions.