Reason/work why Na2O + H2O -> Na+ + OH-?

Answer 1

The sodium ion is 100% soluble in water. Water disassociates into the Hydrogen hydroxide ions. Once the Oxygen from the Na2O has a negative charge, the hydronium Ion is attracted to it.

Answer 2

An acid/base reaction. Remember Na2O is in fact 2Na+ and O(2-). If you look up the acid/base pKa table, you will see that O(2-) is one of the strongest bases, even stronger than OH-. It will pull off a hydrogen (as H+) off from water (which in this case is the acid). So, the equilibrium tends to the right, driving the reaction (curiously, OH- is actually the conjugate ACID of O(2-) here).