Oxygen incorporation is a promising and effective way for simultaneously engineering the edge and planar sites of MoS2 to fabricate high-performance HER electrocatalysts. However, the insulating MoO3 species are inevitably generated during the oxygen incorporation process. Here, controlled plasma exposure time has been designed to explore the influence of MoO3 species on the HER performance for the O-2 plasma modified MoS2 catalysts. We find that the MoO3 species in MoS2 are not conductive to the HER electrocatalysis. More importantly, the Raman spectrum, X-ray photoelectron spectroscopy, energy disperse X-ray spectrum and transmission electron microscope analyses demonstrate that these co-generated MoO3 species are reduced and resolved from the MoS2 lattice during the electrocatalytic hydrogen evolution, leading to a holey structure in the MoS2 nanosheets and thus a significant improvement of its hydrogen evolution stability. This work not only helps to understand the electrocatalytic influence of the co-generated MoO3 species in MoS2, but also presents a methodological improvement in designing defect-rich, oxygen incorporated and holey MoS2 nanosheets for highly efficient hydrogen evolution. (C) 2018 Elsevier Inc. All rights reserved.