In the conversion of  $Cr{O_4}^{2-} \to C{r}_2{O_7}^{2-}$, the oxidation number of oxygen

It is given to find out the change in oxidation number of oxygen during the conversion of$Cr{O_4}^{2-} \to C{r}_2{O_7}^{2-}$

Oxidation number of oxygen remains unchanged that is $-2$ on both sides.

Generally, Oxygen shows an oxidation number of $-2$ except for the peroxides and $O{F}_2.$