The resonating structures of cyanate ion are

$\mathrm{O}=\mathrm{C}=\mathrm{N}⟷{\mathrm{O}}_{-}^{1-}\mathrm{C}\equiv \mathrm{N}⟷\mathrm{N}\equiv \mathrm{C}-{\mathrm{N}}^{2-}$. Th correct set of oxidiation states of O, C and N respectively with the most stable structure out of the above is:

The electron exchanges between O and C is double bond o has (-2) charges. The carbon atom sharing 4 electrons with O and N atom hence there are also 4 valency of electron mean atom has (+4) charges

While N atom sharing one there for there would (-1) charge and -2 charge already present then it will become (-) charge on N atom.

Hence, the oxidization states of O,  C and N is -2, +4, -3 respectively.