We have remember that lattice energy increases as the size of the ions in the ionic solid decreases. According to Oxford dictionary; lattice energy is "a measure of the energy contained in the crystal lattice of a compound, equal to the energy that would be released if the component ions were brought together from infinity."
We know that the larger the magnitude of charge on a cation the smaller in size the cation is and the size of anions increase from left to right in the periodic table. Hence Ca^2+<Na^+ and O^2- < Cl^-. Therefore the lattice energy of CaO is greater than that of NaCl.
