Through first-principles calculations, we systematically investigated the hydrogen interactions with the oxygen-vacancy (O:V) pairs complex in bcc Fe matrix (mimic oxygenenriched nanodusters (NCs) of ODS steels) in comparison with the vacancy-alone defects. The results uncovered that the presence of the (O:V) pairs in oxygen-enriched NCs play a crucial role in prohibiting the growth and swelling of the hydrogen cluster but strongly trap a few hydrogen atoms around each cluster of vacancies. As accompanied with a high density of dispersed NCs in ODS steels, this fact significantly elevates the tolerance of the critical hydrogen concentration of ODS steels as compared with traditional steels. The underlying mechanism to pin the growth of hydrogen cluster has been elucidated to be strongly correlated with the viable charges transfer from the nearby Fe atoms around vacancies. This is the key to determine the trapped concentration and the distribution of hydrogen atoms in ODS steels. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.