Doping of organic semiconductors (OSCs) with transition metal oxides such as molybdenum trioxide (MoO3) has been used as a powerful method to overcome common issues such as contact resistance and low conductivity, which are limiting factors in organic optoelectronic devices. In this study, the mechanism and efficiency of MoO3-induced p-type doping in OSCs are investigated by means of simultaneous electrical and spectroscopic measurements on lateral diodes. It is demonstrated that energetic changes in the MoO3 energy levels outside vacuum can limit charge-transfer doping and device performance. It is shown and investigated that these changes crucially depend on the OSC. The time evolution of important OSC parameters such as induced charge density, doping concentration and efficiency, conductivity and mobility, is deduced. Moreover, the energetic and chemical changes in MoO3 are investigated via ultraviolet and x-ray photoemission spectroscopy. Combining these experiments, important conclusions are drawn on the time-dependence and stability of MoO3-doping of OSCs, as well as on the processing conditions and device architectures suitable for high-performance devices.