The prominent 5-eV absorption band in silicate and germanosilicate glasses has been the subject of extensive investigations by many groups. Several defect models involving oxygen deficiency have been proposed to explain the experimental observations. In particular, careful analysis of the associated photoluminescence bands indicates the presence of more than one component to the absorption band. However, the microscopic defect models responsible for this absorption band are still under debate. In this work, we have investigated the structural and electronic properties of oxygen deficient defects in silicate and germanosilicate glasses by first-principles quantum chemical studies using a cluster approach. In particular, the photoabsorption and photoluminescence properties of several competing defect models have been evaluated. The performance of different quantum chemical methods for the accurate calculation of the optical properties of defects is assessed. The effect of Ge on the relative thermodynamic stabilities of the defects is examined. Careful comparisons are performed with the known experimental observations. New models are proposed to explain the different components of the 5-eV photoabsorption band in glasses.