The OSER project (Optical Scintillation by Extraterrestrial Refractors) is proposed to search for scintillation of extragalactic sources through the galactic -- disk or halo -- transparent $\mathrm{H_2}$ clouds, the last unknown baryonic structures. This project should allow one to detect column density stochastic variations in cool Galactic molecular clouds of order of $\sim 3\times 10^{-5}\,\mathrm{g/cm^2}$ per $\sim 10\,000\,\mathrm{km}$ transverse distance.