We apply first-principles molecular dynamics to investigate the order-disorder transition in the oxygen sublattice, observed experimentally in thoria. The mean-square displacements were calculated at high temperatures (2700K-3660K) and we demonstrate that oxygen atoms become mobile at 3000K, while they stay confined at 2700K. The rate of diffusion of oxygen atoms was calculated while for thorium atoms the mean-square amplitudes of vibrations were evaluated. Thoria is predicted to exhibit a higher increase in thermal expansion with temperatures in the premelting region, similar to urania.