Coking of heavy oil plays a key role in in situ combustion, which is affected by conversion conditions. In this study, a fixed-bed reactor and a pressurized reactor were used to thermally transform a heavy oil sample from China into coke under different atmospheric conditions, heating rates, and pressures. Elemental composition and surface functional groups were studied by an elemental analyzer and diffuse reflectance Fourier transform infrared spectrometer, whereas oxidative activity and coke yield were characterized by a thermal gravimetric analyzer. Results showed that yields, characteristics, and oxidative activities differed between the cokes produced in inert and oxidizing atmospheres. Compared with an inert atmosphere, an oxidizing atmosphere presented lower coke formation temperature and higher yield. In comparison with inert atmosphere, the coke produced in oxidizing atmosphere contained more oxygen, thus increasing the amount of surface functional group, but its oxidative activity was poorer. In oxidizing atmosphere, conversion conditions influenced yield and characteristics of coke but exerted minimal influence on oxidative activity. Oxygen content increased, whereas coke yield decreased with increasing O-2 concentration. Oxygen content decreased, and yield increased initially and then decreased with increasing heating rate. Temperature of coke formation and oxidation decreased, while oxygen content and yield increased with increasing pressure. Through analysis of elements and functional groups of residues produced at different holding temperatures, our study confirmed that the following processes occurred during low-temperature oxidation: evaporation, oxygen-adding reaction, dehydrogenation and dealkylation, polymerization, decarbonylation, and mild oxidation of coke. Conversion conditions, including heating rate, oxygen concentration, and pressure, affected conversion rate of these processes, thus influencing coke characteristics.