To understand the initial steps of the oxidation of Cu(110), we applied density functional theory (DFT) calculations to study oxygen subsurface adsorption at the Cu(110)-c(6 × 2) reconstructed surface by increasing oxygen coverage. A transition from oxygen octahedral occupancy to tetrahedral preference occurs when the coverage reaches 1 monolayer, which may signal the onset of bulk oxidation that initially forms highly distorted Cusbnd O tetrahedrons by comparing the bond lengths and angles of the resulting Cusbnd O tetrahedron with the bulk Cu2O structure. These results suggest that a critical oxygen coverage is required for such a crossover from the oxygen chemisorption to bulk oxide formation. A comparison with the oxygen subsurface adsorption at Cu(100) suggests that the Cu(110) surface has a larger tendency to form Cusbnd O tetrahedrons.We model oxygen subsurface adsorption at the Cu(110)-c(6 × 2) reconstructed surface. Increasing O coverage leads to O tetrahedral preference over octahedral occupancy. The resulting Cusbnd O tetrahedrons show resemblance to that of the Cu2O structure. Cu(110) shows a stronger tendency than Cu(100) to form Cu2O-like tetrahedrons.