Defect structures associated with tin doping of indium oxide, an optically transparent conductor, have been characterized by atomistic simulations and first-principles density functional calculations. A comprehensive survey of defect clusters containing up to three tin dopants in the first and second cationic coordination shells of an oxygen interstitial has been conducted. The analysis of energetically favorable defects gives insights into the role and nature of defect clusters in the material. In particular, the origins of the experimentally postulated b-site preference of tin dopants have been examined. Our results show that b-site preference occurs only in defect clusters with oxygen interstitials and is not intrinsic to dopants. In contrast, in nearest coordination to an interstitial, a strong d-site preference is found. Density functional calculations in the discrete variational-embedded cluster approximation have been conducted on selected defect structures to illuminate the effect of clustering on partial atomic charges, bond-orders, and Sn-119 Mossbauer parameters.