Two oil shale (OS) samples with different organic matter (OM) content from the Khoot deposit, Mongolia were characterized, their pyrolysis and thermal dissolution experiments were carried out and the results compared with those obtained for the Estonian oil shale sample in the same experimental conditions. The thermal decomposition of the organic matter of oil shale samples starts in the temperature range of 200-300 degrees C, while the highest yield of liquid products (tar and pyrolysis water) and the lowest yield of solid residue are obtained at 500-600 degrees C, which can be considered an optimal temperature range for the pyrolysis of OS samples. In this temperature region, 23% of tar is obtained from the Estonian oil shale sample, which is more than the yield from Mongolian Khoot oil shale samples. The higher tar yield from the Estonian oil shale sample is due to its lower ash and higher organic content compared to Mongolian Khoot oil shale samples. The results of the thermal dissolution experiments for the three OS samples show that the yield of liquid product (tar) is the lowest at 350 degrees C, the highest at 400 degrees C and lower again at 450 degrees C.. So, 400 degrees C can be considered an optimal temperature for the thermal dissolution of all the samples in tetralin as a hydrogen-donor solvent. The optimal temperature of thermal dissolution decreased almost 200 degrees C compared to that of pyrolysis due to the use of an autoclave, which is more suitable for carrying out the thermal decomposition of oil shale organic matter than the retort. Also, the yield of tar upon thermal dissolution is higher than that in pyrolysis, because the hydrogen in tetralin molecules favours its formation.