The usefulness of overlap population (OP) diagrams for peak assignments of an electron energy loss near-edge structure (ELNES) and an X-ray absorption near-edge structure (XANES) is demonstrated. Mg-K, L(2,3), and O-K edges of MgO are taken as examples. Theoretical calculations are performed using a first-principles orthogonalized linear combination of atomic orbitals (OLCAO) method. A core-hole is included explicitly, and a large supercell is used to minimize artificial interactions among the core-holes in adjacent cells. All experimental spectra are quantitatively reproduced by the calculations. The OP diagrams for a selected pair of atomic orbitals are computed in order to provide proper assignments for each peak in ELNES and XANES. They are interpreted in terms of interactions among Mg-Mg and Mg-O bonds. Results are found to be consistent to our previous conclusion, which was obtained using a cluster method [T. Mizoguchi, et al., Phys. Rev. B 61 (2000) 2180]. The powerful combination of the OP diagram and a high-energy resolution ELNES to obtain fine electronic structures is also demonstrated.