We present the electronic structure and energetics of atomic oxygen adsorption on graphene-like BC2N honeycomb monolayers in view of density functional calculations using GGA scheme. After the geometry optimization of bare BC2N monolayer, we consider eight initial configuration for binding of atomic oxygen. As initial configurations, atomic oxygen is located approximate to 1 angstrom above the following sites; mid of the B-N, B-C, C-C, N-C bonds, B, N, C atoms, and hollow of the hexagonal cell. Among the bare structures, type-Ills determined as the most stable structure energetically. This can be conceived by visualization of electronic environments of each atom, since type-II geometry optimizes the number of C-C and B-N bonds. When the oxygenated structure is concerned, atomic oxygen is located at the bridge site of B-C bond of type-II geometry yielding most favorable structure energetically. The adsorption energies and reaction paths of oxygen are also investigated. The results show that the oxygenation of type-I structure induces a semiconducting band gap, while semiconducting nature of type-II and type-III is preserved under oxygen adsorption. (C) 2013 Elsevier B.V. All rights reserved.