This paper investigates the effectiveness of oxygen-enriched combustion process at low temperatures to avoid the unburnt carbon that remains in ash during conventional burning process. For this, thermal treatment of low-quality fuels such as olive pomace and Turkish lignite (Afsin-Elbistan) under oxygen-enriched conditions was tested in a tube furnace at temperatures between 400 and 700 degrees C under O-2/N-2 mixtures containing O-2 ratios in the range of 25-50 vol%. The calorific value and the unburnt carbon content of the residues from these tests were used to investigate the combined effects of temperature and O-2 concentration on unusable part of fuels. Thermal reactivity of untreated parent samples and the residues obtained from oxygen-enriched combustion was also compared based on differential thermal analysis (DTA) and derivative thermogravimetry (DTG) profiles. It was determined that oxygen-enriched conditions are able to remove the organic part of the fuels at low temperatures easily as O-2 concentration increases and the oxygen-enriched conditions shifted complete burning temperature to lower values.