The current research involves characteristics of pyrolysis and thermo-oxidation of Estonian oil shale (OS) and its char. Non-isothermal and isothermal thermal analysis methods were used to characterise and compare the specifics of these processes under air and oxy-combustion conditions using Ar, CO2, O2 and their mixtures. The study of reaction kinetics including pyrolysis and oxidation characteristics is crucial to understand the complex mechanism of OS oxy-fuel combustion. The experimental results showed that the pyrolysis behaviour is highly similar in Ar and CO2 until the end of decomposition of organic part. There is no visible char carbon and CO2 reaction during the char oxidation stage under oxy-fuel conditions. The release of CO related to the char carbon and CO2 reaction in 100 % CO2 atmosphere increases notably at temperatures above 650–700 °C. The role of inorganic minerals in the oxidation and gasification of char was described as well as the effect of oxy-fuel conditions on gaseous emissions. It was found that the rate constants for char oxidation in CO2/O2 are approximately 1.2–1.3 times higher as compared to Ar/O2 atmosphere and that char oxidation under oxy-fuel conditions takes place with lower activation energies as compared to Ar/O2.