Remember that Dualistic Theory formulas always involve two oxides. The first oxide is a common metallic oxide, such as CuO. The second is a common nonmetallic oxide, such as SO₃.

I think that you should consider this a little more. You might notice that in Dualistic Theory we always write both parts of a compound as oxides. The first part is a common metallic oxide and the second part is a nonmetallic oxide (usually a common gas). Although CO is a common gas, CaO₂ is not a common oxide.