Precise engineering of the cocatalyst-photocatalyst interface and optimization of the cocatalyst dispersion are essential for improving the activity of particulate semiconductor photocatalysts. Herein, we report the effects of varying the conditions used to load cobalt oxide (CoOx) as a cocatalyst on the O2 evolution activity of a particulate BaTaO2N photocatalyst, based on trials in an aqueous silver nitrate solution under visible light irradiation. Annealing under an N2 flow after loading the Co species increased the O2 evolution rate threefold compared to that obtained following conventional annealing under an NH3 flow. Subsequent annealing under an H2 atmosphere exposed the BaTaO2N surface as a result of the aggregation of CoOx particles, and further enhanced the photocatalytic O2 evolution by a factor of two, yielding an apparent quantum efficiency of 0.55% at 420 nm. These results indicate the importance of intimate contact between cocatalyst particles and the photocatalyst, as well as the necessity of exposing the photocatalyst surface to make it available for reduction reactions during photocatalytic water oxidation.