What is the oxidation number of the oxygen atom in the $O_2^{2-}$ ion?

When $KMnO_4$ is reduced with oxalic acid in acidic solution,the oxidation number of $Mn$ changes from

The oxidation number of nitrogen atoms in $NH_4NO_3$ are

Oxidation number of $N$ in $HNO_3$ is

Iodine reacts with concentrated $HNO_3$ to yield $Y$ along with other products. The oxidation state of iodine in $Y$ is

$LiCoO_2$ crystallizes in a rhombohedral structure. Consider a situation where $50\%$ lithium $(Li)$ is extracted from the lattice. To keep the crystal electrically neutral, the change in the average oxidation state of $Co$ is