Dear student,
the compounds are formed such that more electronegative compound gets negative oxidation state whereas less electronegative compound gets positive oxidation state.
Halogens are less electronegative than oxygen and hence halogens should have positive oxidation state and oxygen should have negative oxidation state.
But as it is seems form the formula of oxygen bromide and oxygen chloride, the oxidation state of oxygen is +2 and that of Br or Cl is -1.
Hence, the given compounds are not stable.
Regards