The low-temperature pyrolysis of oxygen-containing functional groups in coal plays a vital role in coal spontaneous combustion. This paper conducted isothermal oxidation experiments on raw and pyrolyzed coal samples to investigate the influence of the content of oxygen-containing functional groups, pyrolysis temperature, oxidation temperature, particle size and pyrolysis-oxidation times on the oxidation characteristics of pyrolyzed coal samples. It attributes the generation of massive active sites that are oxidized once exposed to O-2 to the pyrolysis of oxygen-containing functional groups in coal. In addition, the oxidation of active sites releases such gaseous products as CO and CO2 in large quantity and producing oxygen-containing functional groups. For a coal sample with higher content of oxygen-containing functional groups and pyrolysis temperature and smaller particle size, the yield of active sites is greater. The formation law of gaseous products during isothermal oxidation after pyrolysis satisfies the exponential attenuation function P = A + Be-t/K1 + Ce-t/K2, suggesting the two reaction processes during the oxidation of pyrolyzed coal, that is, the oxidation of active sites after coal pyrolysis and the subsequent thermal decomposition with the generation of functional groups. The activation energy required for the former is far lower than that for the latter, and the oxidation of active sites can occur at very low temperatures (below 9 degrees C). The results are highly applicable to the study on the mechanism of self-heating and spontaneous combustion of coal.