Oxygen is a unique element, with the only known molecular magnet and with very small compact valence orbitals allowing the short double chemical bond in the oxygen molecule. Remarkably, oxygen exhibits a richer phase diagram than other diatomic molecular systems. To date, the structural investigation of oxygen under pressure has been limited to 133 GPa and the structural characterization of the metallic transition about 100 GPa has been disclosed by single-crystal X-ray microdiffraction. The structural evolution of metallic phase of solid oxygen and the possibility of inducing a molecular dissociation in the solid state remain totally unknown. With this proposal, we will explore to pressures exceeding 300 GPa, by single crystal X-ray microdiffraction, the phase stability of the metallic phase and uncover other possible phases and crystalline structures of very dense solid oxygen.