O2 is considered a molecule. It is made up of two oxygen atoms that are chemically bonded together. This type of molecule is known as a diatomic molecule because it consists of two atoms of the same element.
While O2 is a molecule, it is not considered a compound. A compound is a substance formed when two or more different elements are chemically bonded together. Since O2 consists of only one type of element (oxygen), it does not qualify as a compound.
In summary, O2 is a molecule because it is a group of atoms bonded together, but it is not a compound because it does not involve different elements.