Chapter 4 - Sections 4.1-4.9 - Exercises - Problems by Topic - Page 189: 92e

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

The oxidation state of O is usually $-2$. The sum of the oxidation states of a polyatomic ion is equal to its charge which in this case is $-2$. This means that the oxidation state of Cr is $+6$ as $7(-2)+2(+6)=-2$.