Oxygen has -1 oxidation state in the compound
Caro's acid
Marshall's acid
BaO2
K2O
(1, 2, 3)
Caro's acid H2SO5 contains peroxide group.
Marshall's acid H2S2O5 contains peroxo group. BaO2 is bariumperoxide.
Above three contains peroxo group hence oxygen state of O is -1, whereas in K2O,O.S. is -2.