A strategy of direct growth of Pt on Ni was used to create and control Pt crystal defects on the surface of Ni-Pt core-shell nanoparticles. The control over the types of defects was easily achieved by changing the surfactant system. In this work, two types of crystal defects have been introduced into Ni-Pt core-shell nanoparticles: polycrystalline shells with multiple grain boundaries and step-edge shells with undercoordinated atoms at corners and steps. We show that the step-edge shell has a higher specific activity for the oxygen reduction reaction (ORR), while the thinner polycrystalline shell results in a higher activity per mass and stability. Our results suggest that Ni-Pt core-shell nanoparticles with a thin Pt shell that have high density of crystal defect should be targeted for high performance ORR catalysts.