The impact of tin (Sn) doping on interstitial oxygen (Oi) diffusion in Czochralski silicon is investigated by density functional theory (DFT) calculations. Two aspects are taken into consideration: (i) Sn induced lattice strain and lattice expansion; (ii) direct and vacancy-mediated interaction between Sn and Oi atoms. The calculations show that the interstitial oxygen migration energy barrier increases with increasing lattice constant which grows with the concentration of Sn. Structure analysis indicates that the geometric parameters such as the bond length and the angle of SiOSi configuration also increase with the lattice constant, which is considered to be the origin of the increased energy barrier for oxygen diffusion. SnO pair formation is found to be energetically unfavorable, while the SnVO complexes have a large binding energy. Sn is however able to trap Oi atoms by stimulating vacancy generation around it, which increases the oxygen diffusion energy barrier as well. It is thus predicted that oxygen diffusion in silicon with a high concentration of Sn atoms will be retarded.