Thermoelectric materials convert waste heat into electric energy. Oxyselenide-based materials, specifically, p-type BiCuSeO is one of the most promising materials for these applications. There are numerous approaches to improve the heat-to-electricity conversion performance. Usually, these approaches are applied individually, starting from the pure intrinsic material. Higher performance could, however, be reached by combining a few strategies simultaneously. In the current work, yttrium, niobium, and phosphorous substitutions on the bismuth sites in already bismuth deficient Bi_1-xCuSeO systems were investigated via density functional theory. The bismuth-deficient system was used as the reference system for further introduction of substitutional defects. The substitution with phosphorous showed a up to 40 meV (11 %) decrease in the energy gap between conduction and valence bands at the highest substitution concentration. Doping with niobium led to the system changing from a p-type to an n-type conductor, which provides a possible route to obtain n-type BiCuSeO systems.