Abstract Very recently, the tetragonal BiOCuS was declared as a new superconducting system with a Fe-oxypnictide-related structure. Here, based on first-principle FLAPW-GGA calculations, the structural parameters, electronic band structure, density of states and inter-atomic bonding picture for BiOCuS are investigated. Our results show that, distinct from related metallic-like FeAs systems, the BiOCuS phase behaves as an ionic semiconductor with the calculated indirect band gap at about 0.48 eV. The superconductivity for BiOCuS may be achieved only by doping of this phase. Our data demonstrate that as a result of hole doping, the [CuS] blocks become conducting owing to mixed Cu 3d + S 3p bands located near the Fermi level. For the hole doped BiOCuS the Fermi surface adopts a quasi-two-dimensional-like character, similarly to FeAs SCs.