The correct order of the oxidizing power of Cl2, Br2, I2 and O2 is:
Cl2>Br2>I2>O2
The oxidizing or reducing property of an element is measured by the standard reduction potential values. Greater is the value of standard reduction potential, greater is the oxidizing property of the element. The standard reduction potential values for Cl2, Br2, O2 and I2 are +1.36V, +1.07V, + 0.40V and +0.57 respectively.