that numeral is to show the oxidation state of the compound, and is written in roman numerals to avoid confusion with ionic charge.
the numeral is not needed when writing equations because its replaced by oxidation state.
as for your reaction, you have to consider the substances that u start out with, not the simplest substance.
hence in this case, an O atom generally does not exist in appreciable quantities long enough for a proper reaction, but instead we use oxygen gas, O2.