Water electrolysis has received great attention for producing hydrogen, but sluggish kinetics of oxygen evolution reaction (OER) has remained a big challenge. Recently, cobalt sulfide materials have been widely explored as great choice in highly efficient electrocatalysts due to their good electrical conductivity and bi-functionality toward OER and hydrogen evolution reaction (HER). However, cobalt sulfide shows outstanding HER activity, but its OER activity should be improved. Herein, hexagonal-phase cobalt sulfide (CoS) nano-wires with abundant stepped surfaces and defect sites were prepared via a seed-growth approach with silver sulfide (Ag2S) nanoparticles. The Ag2S-CoS hetero-nanowires (HNWs) exhibited excellent electrochemical performances for oxygen evolution reaction (overpotential = 275 mV, Tafel slope = 77.1 mVdec(-1), charge transfer resistance = 1.3 Omega) in 1.0 M KOH solution. The origin of superior activity was investigated using a combined experimental and theoretical approach. This work highlights the importance of surface defects for improving oxygen evolution reaction performance of water electrolysis.