Freeform optics enable improved optical solutions but their fabrication usually requires complicated precision machining processes. We report on an approach for freeform shaping of optical surfaces via a stress-induced viscous deformation of glass plates. We studied the deformation of fused silica substrates covered by specifically laser patterned films of substoichiometric silicon oxide during annealing at about 1100 °C in an oxidizing ambient. The obtained large deformation of the substrates can be understood by a mostly viscous deformation but can be described in analogy to a purely elastic deformation. Our results demonstrate the feasibility of a method for freeform shaping of individual optical substrates that only requires the preparation of a flat surface.