Chapter 4 - Sections 4.1-4.9 - Exercises - Problems by Topic - Page 189: 91f

$Cr: +6$ , $O: -2$

O normally has an oxidation state of $-2$ and there are four O atoms so their sum is $-8$. The polyatomic ion has an overall charge of $-2$ so the sum of the oxidation states must equal $-2$. This means the oxidation state of Cr is $+6$ as $4(-2)+(+6)=-2$