Oxygen evolution reaction is a critical reaction for renewable energy technologies. Conventionally, metal phosphides or metal sulfides are active for HER, their OER activity is relatively poor. Herein, we have precisely prepared quaternary (NiFe)PS3 nanosheets as ultra-effective OER catalysts via a one-step sulfurization and phosphorization method with NiFe prussian blue analogues as precursors. The systematic results demonstrate that the coexistence of PBA, P and S sources is the keypoint for the nanosheet structure. The (NiFe)PS3 nanosheets show very good OER activity with a super-low overpotential (223 mV) to reach 10 mA cm-2. However, from the energy diagrams, we find that the basal plane of (NiFe)PS3 is almost inert to OER due to the prohibitively high theoretical overpotential of 1.50 V, derives from the weak binding of OH* and OOH* intermediates, and only unsaturated Fe and Ni atoms locate on the (010) plane of (NiFe)PS3 are active for OER. The further ex-situ characterizations demonstrate that the in-situ electrochemically formed oxohydroxide layer on the nanosheets contributes significantly to the OER actvity. This work not only provides a simple method to synthesize quaternary bimetallic phosphosulphide nanosheets, but also provides a deep understanding on the origin of the high activity of (NiFe)PS3 nanosheets.