The oxyanion of chlorine that contains the most oxygen atoms is perchlorate, with the formula ClO₄⁻.

Here's why:

* Hypochlorite (ClO⁻) has one oxygen atom.

* Chlorite (ClO₂⁻) has two oxygen atoms.

* Chlorate (ClO₃⁻) has three oxygen atoms.

* Perchlorate (ClO₄⁻) has four oxygen atoms.