This work investigated the kinetic parameters of the thermal decomposition of Estonian kukersite oil shale (OS) organic part in air atmospheres at various oxygen-nitrogen ratios. During oil shale combustion, two combustion phases were recognized but could not be separated. Thermogravimetric analysis (TGA) of oil shale combustion was conducted in nitrogen-based gases at different oxygen concentrations (5-50% O-2) and heating rates (1, 10, 30 and 50 K/min). The authors modeled oil shale devolatilization and char combustion at different oxygen concentrations by using a discrete activation energy model. The process could be described by four parallel independent reactions. The activation energies were 105134 kJ/mol. The combustion rate was found to be dependent on oxygen partial pressure. The power variables of the oxygen concentration for the reaction models were optimized and compared against a unity base case. Using these data, oil shale devolatilization and char combustion in nitrogen based environments were modeled.