An optical system for a motor vehicle, in particular for an indicating and/or lighting device, arranged to emit a light, the intensity of which is maximal in a given prioritized optical direction. The optical system comprises a surface light source exhibiting a front wall emitting a light, the intensity of which is maximal in a principal direction of emission, and an optical guide comprising an input face and an output face forming at least partly an outer face of the system, the system being arranged such that the light penetrates the guide through the input face and escapes through the output face. The optical guide being conformed such that the intensity of the light at the output of the system is maximal in a prioritized optical direction.