Developing an efficient electro-catalyst for water oxidation is essential for improving the performance, which holds the key for a number of energy conversion and storage devices. Here we report an effective method for fabricating a Ni-Co-O@Ni-Co-S hierarchical nanoarray, which showed a significantly improved activity relative to Ni-Co-O nanowire arrays for oxygen evolution reactions. The enhanced performance was attributed to the secondary formed Ni-Co-S nanoplatelets which not only acted as efficient electrocatalysts, but also facilitated the electrolyte penetration and increased the surface area.