A transparent diffusive OLED substrate includes the following successive elements or layers: a transparent flat substrate made of mineral glass having a refractive index of between 1.45 and 1.65, a rough low index layer including mineral particles, the mineral particles being bonded to one side of the substrate by means of a low index enamel, the mineral particles near, at or protruding from the enamel's surface creating a surface roughness characterized by an arithmetical mean deviation Ra comprised between 0.15 and 3 μm, the mineral particles and enamel both having a refractive index of between 1.45 and 1.65; a high index planarization layer made of an enamel having a refractive index comprised between 1.8 and 2.1 covering the rough low index layer (b).