Which one of the following compound is not made up of charged ions

NaCl, $\text{MgC}{l}_2$ and CaO are ionic compounds.  Hence, they have charged ions like cation and anion $({\text{Na}}^{+},\text{C}{l}^{-})$.

But, the molecule ${\text{O}}_{\text{2}}$ is a covalent nature.  Hence, there are no ions in it.