At 0 K, the oxygen interstitial is the lowest energy defect for all structures with a small negative formation energy for La4Ni3O10 (-0.127 eV) and Pr4Ni3O10 (-0.041 eV) and a small positive formation energy for La2Pr2Ni3O10 (0.067 eV). The formation energy for oxygen vacancies is above 1.5 eV for all sites indicating that there is a strong preference for interstitials in the (M)4Ni3O10 structure at low temperatures.