Using computer simulation techniques the defect structure and oxygen ion migration mechanism of oxide pyrochlores (eg. Gd2Zr2O7) was investigated in order to explain the decreased activation enthalpy for oxygen ion conductivity as a function of order. Shell model potentials were found to be necessary in order to obtain sufficiently accurate physical properties for the pyrochlore compound. The oxygen Frenkel defect consisting of ‘a split 48f vacancy’ and 8b interstitial appeared to be the most stable instrinsic defect, but vacancies related to extended defect structures may play an important role in the diffusion mechanism too. The migration mechanism of oxygen ions is mainly based on 48f-48f jumps and involve 0.9 eV barrier energy, comparable with the experimental activation enthalpies of 70–85 kJ/mol.