In what case does a polyatomic ion lose one oxygen? (-ite atoms)

Answer 1

In the case of reduction during an oxidation/reduction reaction. An example would be chlorate (ClO₃^-) -> Chlorite (ClO₂^-). In ClO₃^-, the Cl has an oxidation state of 5+ and in ClO₂^-, the chlorine has an oxidation state of 3+. The chlorine atom has been reduced.