Which of these two ionic compounds has the higher melting point: sodium oxide or aluminium oxide? Why?

I suppose it is the $\text{alumina}$.
For sodium chloride, each of the constituent ions have unit charge. For alumina, the ionic lattice is composed of $A {l}^{3 +}$ and ${O}^{2 -}$ ions. Because the electrostatic force of attraction should be greater we would predict that alumina should have the greater melting point. The aluminum cation is also smaller than the sodium cation and thus more polarizing.