Density functional calculations are carried out on divacancy-oxygen (V2O and V2O2) complexes in silicon, paying particular attention to their formation and dissociation mechanisms as well as their electrical activity. The formation of V2O around 220°C is controlled by the diffusion of V2 to immobile oxygen traps, while it dissociates around 300°C into VO and V. V2O and V2O2 are found to possess deep single and double acceptor levels as well as deep donor levels similar to those of V2.