Answer
$C: +4$ , $O: -2$
Work Step by Step
O normally has an oxidation state of $-2$ and the are three of them so the sum is $-6$. The polyatomic ion itself has a charge of $-2$ so the sum of all the atoms oxidation states must equal $-2$. This means C must have an oxidation state of $+4$ as $(+4)+3(-2)=-2$