High specific surface area nanoporous Pt-Ni synthesized via selective dissolution of Ni exhibits a high catalytic activity toward the oxygen reduction reaction (ORR). Using kinetic Monte Carlo simulations, we show that the elemental distribution in nanoporous Pt-Ni is different from the well-understood Pt-skin Pt-Ni catalyst. In nanoporous Pt-Ni, both the surface and subsurface layer are rich in Pt. Crucially, a small amount of Ni is present in the surface layer. Using density functional theory, we show that the presence of Ni in the surface layer in nanoporous Pt-Ni can speed up the ORR by orders of magnitude. Different Pt-Ni atom arrangements in the surface layer are considered. Our simulations show that the OH formation step is the rate-limiting step for the ORR The low concentration of surface Ni explains why nanoporous Pt-Ni provides only 10 times improvement in ORR rates over the pure Pt catalyst.