Oxygen (O2) is an element, it consists of only one substance = oxygen
Carbon dioxide (CO2) is a compound it consists of carbon and oxygen
Oxygen is both a compound and an element. In nature, Oxygen forms bonds with itself to form a compound consisting of two elemental oxygen atoms to form one compounded oxygen molecule. This is O2.
Mercuric oxide is a compound, not an element. It is composed of the elements mercury and oxygen.
Iron oxide is not a single element, but rather a compound composed of iron and oxygen atoms.
Water is the compound that contains oxygen and sodium is a metal element. Iron is a metal element as well.
Sodium hypochlorite is a compound, not an element. It is composed of sodium, chlorine, and oxygen atoms.