Smoluchowski’s theory for orthokinetic coagulation of spheres is extended to aggregation during precipitation of calcium oxalate monohydrate. Physical models for hydrodynamics and deposition ill a binary aggregation event are combined to determine the success of collisions with different orientations to the shear field. Consequently, an efficiency term is introduced that represents the proportion of collisions resulting in aggregates. The model is used to investigate the effect of shear rare : crystal size and ion concentration on the aggregation rate constant, beta(agg). A maximum in beta(agg) is observed with shear rate that corresponds to two opposing effects; increasing rate of collisions and increasing tensile stress between crystals. By contrast Smoluchowski’s theory predicts direct proportionality. Compared with Smoluchowski’s theory, beta(agg) is relatively insensitive to crystal size. Finally, beta(agg) is highly sensitive to solution ion concentration, an effect Smoluchowski’s theory does not consider.