The effects of pressure and oxygen concentration on the coal combustion characteristics and ash formation with mineral transformation in a pressurized O-2/CO2 system were investigated. Thermogravimetric experiments indicated that the heterogeneous ignition at atmospheric pressure converted to homogeneous ignition at low and medium pressures, then began to change to heterogeneous ignition gradually at 4 MPa, and finally became heterogeneous ignition at 6 MPa. The increase of oxygen concentration did not affect the ignition mechanism of pulverized coal at 6 MPa, it only enhanced the intensity of combustion. The ignition and burnout temperatures did not change linearly with the increase in pressure, which led to a parabola trend of the combustibility index S. The scanning electron microscopy (SEM) micrographs suggested that more small ash particles were produced during combustion with the increase in pressure, which were possibly a result of the significant differences in char structure. X-ray diffraction (XRD) analysis of the ash samples showed that the pressure had an effect on the mineral phases formed in the residue ashes due to the impact of pressure on the combustion temperatures of different ignition mechanisms.