Which of the following is represented by the abbreviation "O2"?

The abbreviation "O2" refers specifically to oxygen, which is a diatomic molecule consisting of two oxygen atoms bonded together. This molecular form is what makes up the majority of the Earth's atmosphere and is essential for the respiration of aerobic organisms. Oxygen is crucial for various biological processes, including cellular respiration, where it helps convert glucose into usable energy.

In contrast, the other options represent different concepts. Ozone, for example, is represented by the abbreviation "O3," which indicates that it consists of three oxygen atoms. Oxide refers to compounds formed between oxygen and other elements, which are not indicated by the "O2" notation. Osmosis is a biological process relating to the movement of water across a semipermeable membrane and does not involve a chemical abbreviation like "O2." Therefore, the correct identification of "O2" as oxygen reflects its fundamental role in life and chemistry.