Sodium oxide has a formula of Na2O. Since sodium is a metal and oxygen is a non metal, sodium oxide is an ionic species. Sodium will lose an electron to form a cation and oxygen will gain 2 electrons to form anion. These ions will be arranged in an ionic lattice structure. There is a strong attractive force between these oppositely charged ions, which require a lot of energy to break lots of these bonds to melt the lattice structure. This is why sodium oxide has a high melting point.