What is the name is the ionic compound cao
1 calcium oxygen
2 calcium oxide
3 calcium (i) oxide
4 calcium (ii) oxide

Almost without exceptions, the alkali metals (Li, K, Na,...) are in oxidation state (I) and the alkaline earth metals (Be, Mg, Ca, ...) are in oxidation state (II).

So without confusion, the naming of the metal part is simply the metal name, such as calcium oxide.

The Stock system (which indicates the oxidation state) is required only when the metal exhibits multiple oxidation states, such as iron (II) oxide or iron (III) oxide.  

Then again, older nomenclature applies the ending -ous to the lower oxidation state, and -ic to the higher.  Using the Latin name ferrum for iron, the previous compounds were named ferrous oxide and ferric oxide respectively.