Oxygen vacancies on transition metal oxide surfaces are catalytically very important. The stability, shape and replenishing process of the vacancies are critical to understanding reactions happening on the surfaces. In this paper we investigate the stability of various defective MoO3 (0 1 0) surfaces and examine the influence of environmental oxygen on the stability as well as the active sites for the replenishing process. Our calculations reveal that the line oxygen defect along a (asymmetric oxygen) direction is thermodynamically most favorable at higher defect concentration whereas point defect surfaces are unfavorable. Under normal experimental conditions the perfect surface dominates the MoO3 (0 1 0). We show that for stoichiometric surfaces of any oxides (A(x)O(y)) the formation energy per vacancy controls the favorable defect shape (line or point defects). Calculations indicate that O-2 can dissociate readily on the surfaces that double vacancies share one Mo atom. The replenishing process of the oxygen vacancies through O-2 dissociation most likely occurs on the double-vacancy containing one terminal and one asymmetrical oxygen vacancies. (C) 2015 Elsevier B.V. All rights reserved.