Ab initio investigation of the possibility of oxygen ion migration in non-stoichiometric bismuth titanate pyrochlore Bi1.5Ti2O6.(25) has been performed. Density functional theory calculations together with the nudged elastic band method were used to estimate the O2- migration barriers. The migration energy barriers for the most favorable paths of oxygen migration related to O-48f-O-48f jumps are in the range 0.67-0.92 eV with the corresponding vacancy formation energy in the range 1.76-2.60 eV. These paths form extensive migration network in non-stoichiometric Bi1.5Ti2O6.(25) which is sufficient for 3-dimensional O2- diffusion. The results show a good agreement with the experimentally obtained activation energy of oxygen diffusion in doped bismuth titanate pyrochlores with low dopant concentration.