Binary cobalt and nickel phosphides have been widely developed owing to their remarkable activities for the oxygen reduction reaction (OER) as well as their low cost. However, the OER active sites of binary cobalt and nickel phosphides are still controversial. Here, we use a CoNiP nanocage as a model catalyst and systematically investigate the correlation between the composition and the OER activity, clarifying how the ratio of CoOOH/NiOOH affects the OER activity. With the increase of the atomic ratio of Ni/Co (y/x, 0 < y/x < 3.5), the amount of NiOOH generated during the OER always increases; and the CoOOH initially increases and subsequently decreases, showing a similar changing tendency to the OER activity of CoNiP. When y/x = 1.5, CoNiP has the best OER activity with an overpotential of 278 mV at a current density of 10 mA cm⁻² and a low Tafel slope of 67 mV dec⁻¹. All tested CoNiP catalysts show better catalytic activity than pure CoP, indicating that the catalytic activity of CoNiP should be attributed to the synergistic effect of CoOOH and NiOOH rather than exclusively to CoOOH or NiOOH. This study clarifies the origin of the catalytic activities of CoNiP, helpful for designing high-efficiency CoNiP catalysts.