A structural phase transition in Li2O, from the anti-fluorite to the anti-cotunnite structure, is evidenced by angle-dispersive synchrotron X-ray powder diffraction and supported by ab initio calculations. The equations of state E(V), P(V) and the enthalpies H(P) are determined, for both phases, using total-energy calculations based on the Projector Augmented Waves method. The calculations supply the value of the transition pressure at T = 0 and in the ideal conditions and provide detailed information on all 9 structural parameters describing the anti-cotunnite structure of Li2O at various pressures. We also report Raman frequencies, calculated for both phases and measured in the anti-fluorite phase, so as to provide a tool for monitoring the approach and the progress of the transformation.