An interatomic potential is proposed for modelling the thermodynamic, transport, and kinetic properties of microdefects formed in silicon crystals by oxygen in combination with interstitials and vacancies. The potential is based on a combination of the Stillinger-Weber potential for pure silicon and the potential of van Beest, Kramer and van Santen for silicas. The new potential combines these two potentials using the concept of distributed electronic charge between neighboring silicon and oxygen atoms to account for the mixture of ionic and covalent character of these bonds. The potential is fit to first principle calculations for the structure of a single oxygen interstitial in silicon, to the frequency of the infrared absorption peak at 1106 cm(-1), and to the enthalpy of formation for this defect, as predicted from solubility measurements for oxygen in silicon. All features of the puckered bond-center configuration and the absorption peak are reproduced to within 10% accuracy. The potential is used to compute the structure, infrared absorption frequencies and formation enthalpies associated with binary defects formed from oxygen-vacancy and oxygen-interstitial pairs. These results are compared to experimental evidence for these defects.