The water splitting cell converts electrical energy into chemical energy as a hydrogen. Oxygen evolution reaction (OER) occurs on the anode of water splitting cell, and OER has a high overpotential which limits the commercialization of the cell. Nanoporous structures have not been studied intensively for OER electrodes, despite its high surface area which is suitable for OER. In this study, we observe the enhanced OER activity in acidic media on the flat porous platinum layers prepared by electrochemical deposition. The thickness and surface area of porous layers increase without structural changes on the surface by increasing the deposition charge. More importantly, the increase of electrochemical surface area (ESA) is proportional to the decrease of the OER potential. We obtained the lowest OER potential at 10 mA cm-2 as 1.62 V (vs. RHE). In order to monitor the effect of pores on the OER activity, we also measured the OER activity of the electrochemically worn porous layers, and another platinum nanostructure with similar ESA to porous layers. These results suggest that the pores can directly affect OER activity.