Elements in the same column (like Li and K) have the same oxidation numbers (also called oxidation states or charges).

Transition Metals: transition metals do not have consistent oxidation numbers, so we use Roman numerals to notate the oxidation numbers.

For example:
Fe (II) is Fe²⁺ (or Fe⁺²);
Fe (III) is Fe³⁺ (or Fe⁺³);
Cu (II) is Cu²⁺ or (Cu⁺²).

You can practice on this exercise or learn them from the song "Oxidation Information".