The doping effects of the boron group (Al, Ga, In and Tl) and the nitrogen group elements (As and Sb) on BiCuSeO are studied combining a band unfolding technique and density functional theory. Substitutional site preferences of these dopants are predicted based on the formation energy. It is found that significant resonant states near the conduction band minimum and valence band maximum of BiCuSeO are induced by In and Tl, respectively, providing a guideline for the enhancement of thermoelectric efficiency through band engineering. Effective band structures of the doped systems have been obtained for direct examination of the resonant states. Arsenic tends to substitute either Bi or Se atoms, while the electronic structure strongly depends on the substitutional sites. The decomposed density of states and charge densities are also calculated to unveil the origins of the resonant effects.