Since potassium forms only one ion, #"K"^+#, no Roman numeral is used. Oxygen forms two ions; the oxide ion, #"O"^(2-)#, and the peroxide ion, #"O"_2"^(2-)#.
The formula units of nearly all ionic compounds are also the empirical formulas, as they represent the lowest whole-number ratios of the ions in the compound, though there are exceptions. KO is one of those exceptions.
The formula unit for potassium oxide is #"K"_2"O"#, which is also the empirical formula. The formula unit for potassium peroxide is #"K"_2"O"_2#. The ratio of #"K"^(+)"# ions to #"O"_2"^(2-)# ions is #"1:1"#, so #"KO"# is the empirical formula for potassium peroxide.
The reason that the formula unit for potassium peroxide is not reduced to #"KO"#, is that the peroxide ion has the formula #"O"_2"^(2-)#. It is composed of two covalently bonded oxygen atoms plus two extra electrons. Therefore, the peroxide ion cannot be further reduced.
