The oxidation state of any element in its free (by itself or with atoms of same element) state is ##0##.
To start off, I’ll write the rules for assigning :
Since the element Aluminium is by itself, the rule number 1 applies. The oxidation number on it is ##0##.
However, if the aluminium was to react with oxygen to form Aluminium Oxide (##Al_2O_3##), the oxidation number for aluminium would be ##+3## since oxygen is ##-2## (rule 5) and there are 3 oxygen atoms in a molecule so each aluminium atom must be ##+3## charge to satisfy rule 3.