Here's why:

* Electronegativity is the ability of an atom to attract electrons towards itself in a chemical bond.

* Oxygen has a higher electronegativity value (3.44) than chlorine (3.16) on the Pauling scale. This means oxygen has a stronger pull on electrons in a bond compared to chlorine.

Here's a simple analogy: Think of a tug-of-war. Oxygen is a stronger competitor and would be more likely to pull the "electron rope" towards itself.