P-type metal oxide semiconductors have been developed for solar fuel generating systems, but efforts to improve energy efficiency remain challenging due to the limited understanding and control of defects. Herein, p-type CuBi2O4 was chosen as a prototypical model system to investigate the presence and mitigation of inevitable defects, including hydrogen impurities and oxygen vacancies. By a combination of experimental and theoretical analyses, these defects were determined to be shallow donors which reduce p-type conductivity as well as introduce defect levels at the surface, which are correlated with increased recombination. Using thermal treatment in an oxygen atmosphere, both defect types were reduced in concentration, resulting in an improved onset potential of 270 mV, 3.9 times enhanced photocurrent with decreased recombination, and a longer photocarrier lifetime. This work aims to provide a broader understanding of hydrogen impurities and oxygen vacancies in p-type metal oxide semiconductor photoelectrodes to further advance their practical application.