[en] The behavior of solid oxygen in the pressure range between 5-116 GPa is studied by ab-initio simulations, showing a spontaneous phase transformation from the antiferromagnetic insulating δ-O₂ phase to a non-magnetic, metallic molecular phase. The calculated static structure factor of this phase is in excellent agreement with X-ray diffraction data in the metallic ζ-O₂ phase above 96 GPa. We thus propose that ζ-O₂ should be base centered monoclinic with space group C2/m and 4 molecules per cell, suggesting a re-indexing of the experimental diffraction peaks. Physical constraints on the intermediate-pressure ε - O₂ phase are also obtained. (author)