The effect of oxygen doping on the electronic and geometric structures of monolayer graphitic carbon nitride was calculated by first principle. It reveals the favorable 0 doping configurations over all the Fermi levels utilizing the Ab initio thermodynamics approach. The valence charge density difference contour map presents a weaker covalent nature on O-C bonds for O-N2-doped structure and a complex ionic-covalent character associated with O-N2 bonds for O-i-doped structure. Based on the analysis of the electronic structures of the doped and un-doped systems, it is found that O doping facilitates the visible-light absorption of monolayer g-C3N4. Especially, O-i doping shows an intrinsic semiconductor behavior and the occupied doping band can be avoided to be the recombination center. In addition, O doping causes slightly stronger delocalization of the HOMO and LUMO which facilitates the enhancement of the carrier mobility. Moreover, O-i doping can induce more activity sites, and, thus, is beneficial for the separation of photogenerated e(-)/h(+) pairs to some extent. (C) 2015 Elsevier B.V. All rights reserved.