The response of MgO periclase to irradiation is investigated by means of molecular dynamics simulations, mimicking irradiation by Frenkel pairs accumulation. Both calculated lattice and volume swellings which refer respectively to lattice and total volume changes reproduce well the experimental measures. The two diverge around 0.2 dpa, above which lattice and volume swellings follow separate trends. Below this value, dislocation loops nucleate from point defects clusters, built up by progressive aggregation of both magnesium and oxygen interstitials. Very small ½<110> loops lying in {001} planes and made of (MgO)6 interstitials could be characterized. They serve as seeds for the subsequent growths of dislocation loops in all three {110}, {001} and {111} planes, obeying then a sublinear law. The ½<110> loops lying in the {011} planes become dominant as loop diameters increase beyond 15 nm. Above 0.2 dpa, it is observed (i) the relative decrease of lattice swelling mainly because the very dense dislocations loops recombine and stabilize into less dense dislocation forest and concomitantly (ii) the fast amplification of volume swelling caused by the occurrence of significant voids of up to 32 vacancies.