A base oxide soluble in water?

All oxides of Group I metals are soluble in water by reacting with water to form soluble hydroxides. The equations are :
Li₂O(s) + H₂O(l) → 2Li⁺(aq) + 2OH⁻(aq)
Na₂O(s) + H₂O(l) → 2Na⁺(aq) + 2OH⁻(aq)
K₂O(s) + H₂O(l) → 2K⁺(aq) + 2OH⁻(aq)
Rb₂O(s) + H₂O(l) → 2Rb⁺(aq) + 2OH⁻(aq)
Cs₂O(s) + H₂O(l) → 2Cs⁺(aq) + 2OH⁻(aq)

Beryllium oxide and magnesium oxide are insoluble in water, and all other oxides of Group II metals are soluble in water by reacting with water to form soluble hydroxides. The equations are :
CaO(s) + H₂O(l) → Ca²⁺(aq) + 2OH⁻(aq)
SrO(s) + H₂O(l) → Sr²⁺(aq) + 2OH⁻(aq)
BaO(s) + H₂O(l) → Ba²⁺(aq) + 2OH⁻(aq)

All other base oxides are insoluble in water.

Sodium Oxide (Na2O)
Potassium Oxide (K2O)