Considering the results of baryonic compact massive ob jects searches through microlensing [1], cool molecular hydrogen (H_2 - He) clouds should now be seriously considered as a possible major component of the Galactic hidden matter. It has been suggested that a hierarchical structure of cold H_2 could fill the Galactic thick disk [8] or halo [3], providing a solution for the Galactic hidden matter problem. This gas should form transparent "clumpuscules" of ~ 10 AU size, with a column density of 10^ 24-25 cm^-2 , and a surface filling factor smaller than 1%. The OSER project (Optical Scintillation by Extraterrestrial Refractors) is proposed to search for scintillation of extra-galactic sources through these Galactic - disk or halo - transparent H_2 clouds. This project should allow one to detect (stochastic) transverse gradients of column density in cool Galactic molecular clouds of order of ~ 3 × 10^-5 g/cm^2 /10 000 km. We present the expectations from a complete simulation of the interstellar scintillation process. Test observations have been obtained through known nebulae to probe the concept in a controlled situation; we show preliminary results from their analysis.