The GaxO (x = 2–4) clusters were studied using density functional theory (B3PW91). The global minima contain linear dicoordinate, T-shape tricoordinate and planar tetracoordinate oxygen for Ga2O, Ga3O and Ga4O, respectively. The 18-electron rule and preference for planar structure for Ga4 contribute to square structure for Ga4O. NICS values reveal that the aromaticity (especially π aromaticity) contributes to stabilizing the planar tetracoordinate bonding of oxygen. In addition, single coordinate terminal oxygen has been found to be most unstable in gallium and oxygen system because of high energy.