Oxidation number of oxygen in ozonides is

 In Ozonides, the oxidation number of oxygen is $-\frac{1}{3}$. The reason is the amount of uni negative charge on ${\mathrm{O}}_3^{-}$ ion must be sharing by 3 oxygen atoms. Hence, the oxidation number of oxygen in ozonides to be $-\frac{1}{3}$.