Oxidation number of osmium $(Os)$ in $OsO_4$ is

Identify the process in which the change in the oxidation state is $5$:

What is the oxidation number of phosphorus in calcium phosphate?

$3 \ mole$ of $N_2H_4$ loses $30 \ mole$ of electrons to form a new compound. Calculate the oxidation state of $N$ in the new compound,assuming the oxidation state of $H$ remains unchanged.

When $K_2Cr_2O_7$ is converted to $K_2CrO_4$,the change in the oxidation state of chromium is

What is the oxidation state of oxygen in $OF_2$ and in $KO_2$ respectively?