Balanced chemical reaction: 2KClO₃ → 2KCl + 3O₂.
Potassium is metal from first group of Periodic table of elements, it has always oxidation number in compound +1.
Chlorine in the reactant state has oxidation number +5 (+1 + x + 3 · (-2) = 0) and it is reduced to -1 (+1 + x = 0) in the product state.
Oxygen in the reactant state has oxidation number -2 (+1 + 5 + 3 · x = 0) and it is oxidized to 0 (in molecule of oxygen) in the product state.