Ab initio total-energy calculations are performed to investigate the initial stages of oxygen aggregation in silicon. Since the volume per SiO2 unit in silicon dioxide is approximately twice the volume per silicon atom in crystalline silicon, it is generally believed that oxygen clustering in silicon will require the creation of self-interstitials. In a similar vein, it is generally believed that neighboring interstitial oxygen configurations are unbound in a weakly distorted silicon lattice. In this work, several interesting phenomena that are associated with aggregation and do not require the creation of silicon self-interstitials are predicted. These predictions are (1) oxygen atoms can cluster with large binding energies of about 1 eV; (2) this clustering can only occur for very specific geometries; and (3) the geometries are chainlike arrangements of oxygen bridging configurations. Indeed, it is the quasi-one-dimensional nature of the proposed oxygen clusters that allows the complexes to form without the accompanying large lattice stress that can lead to the ejection of silicon interstitials. The thermal treatment necessary for aggregation is identified, and predictions of the observable effects are made. © 1991 The American Physical Society.