The synthesis of BiAgOCh (Ch = S or Se) compounds has been successfully achieved via the ion exchange of copper with silver in aqueous solutions, starting from the copper parent phase. Optical and electrical measurements of BiAgOCh powders confirm an increase in both the bandgap and the electrical resistivity, as compared to those of the copper compounds. The structure of the BiAgOS phase has been clearly examined. X-ray diffraction synchrotron measurements coupled with advanced high-resolution transmission electron microscopy analysis evidenced a Ag-deficient structure, as well as Bi-rich defects, both types of defects being oppositively charged. Silver atoms are also found in interstial sites, which explains the two-dimensional ionic conductivity. This structural study combined with theoretical calculations explains the intrinsic conductivity behavior of these semiconductors linked to the mutual compensation of both defect types in the structure and to the increase in the hole effective mass. This study shows the feasibility of modifying the optoelectronic properties of the BiMOCh compounds, with the goal of integrating them in heterojunction solar cells. Moreover, it provides very precise insight into the complexity of the relationship between structural defects and optoelectronic properties.