Oxygen-enriched air combustion of low-grade fuels with high inorganic matter is of great interest due to the efficient burning of such fuels and mitigation of emissions. For this purpose, this study aims to investigate the combined effects of oxygen enrichment and temperature on burnout levels. The oxygen-enriched air combustion performance of two Turkish lignites with different geological ages (early Miocene-Pliocene period Kutahya-Tuncbilek (KT) lignite and Pleistocene period Adiyaman-Golbasi (AG) lignite) was investigated in a horizontal tube reactor. The lignite samples were heated slowly (10 °C/min) to the temperatures of 200–600 °C for AG lignite and 200–800 °C for KT lignite under N2/O2 atmospheres with O2 ratios of 21, 30, 40, and 50 vol%. The solid residue remained after this oxidative heat treatment was characterized by proximate/ultimate analyses, higher heating value, (HHV) thermal analysis, Fourier transform infrared spectroscopy, (FTIR) X-ray diffraction (XRD), and scanning electron microscopy (SEM) techniques. It was concluded that AG lignite that is relatively younger lignite is more susceptible to the O2-enriched conditions as the treatment temperature or O2 concentration increases. It was also determined that the combined effects of temperature and O2 concentration are much more profound than the individual effects of these parameters.