Which medical abbreviation represents "oxygen"?

The abbreviation "O2" is the correct representation for oxygen. This notation comes from chemical nomenclature, where the subscript "2" indicates that each molecule of oxygen consists of two oxygen atoms. This form (O2) is essential because oxygen is typically found in the molecule form of two atoms in the air we breathe.

The other options do not correctly represent oxygen in this context. "O3" refers to ozone, which consists of three oxygen atoms and is a different molecule with distinct properties and functions. "Ox" is a shorthand that might imply oxygen in some casual contexts, but it is not a standardized medical abbreviation. The standalone "O" does not convey the necessary detail as oxygen, it is too vague and does not follow the standard representation in medical terminology.