The complex $\ce{K6[(CN)5Co-O-O-Co(CN)5]}$ is oxidised by bromine into $\ce{K5[(CN)5Co-O-O-Co(CN)5]}$.

In this we have to comment on the change on length of $\ce{O–O}$.

But for that we have to find the oxidation number of $\ce{O2}$, so that we can know bond order.

But I am confused, how can I find the oxidation number?