In this paper we report a neutron total scattering experiment coupled to pair distribution function analysis (PDF) on a recently discovered Ga-based oxide-ion conductor characterized by interstitial oxygen migration. In particular, this study focuses on the short-range order analysis of the defect structure in La1.50Sr0.50Ga3O7.25 with the aim of shedding light on the position of the interstitial oxygen and on the distortion induced in the structure by the extra-oxygen. The data analysis reveals that the preferred position, at low-temperature, for the interstitial oxygen is within the pentagonal ring formed by the Ga1 and Ga2 atoms and shifted with respect to the centre of the pentagon. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.