The emission characteristics of non-premixed oxy-fuel and oxygen-enhanced flames in burned gas entrainment above the self-ignition temperature (BEST) conditions were experimentally investigated in a lab-scale slot-type combustor. The inside geometry of the furnace was width = 250 mm, depth = 250 mm, and height = 350 mm. A slot-type burner was located on the center of the furnace bottom. The widths of the fuel jet and oxidizer nozzle exit were 0.24 mm and 0.52 mm. The separation distance between the nozzle exits was 8 mm and the depth of the nozzle exit was 20 mm. Light emission data were obtained with a digital single lens reflex camera, spectrometer, and intensified charge-coupled device camera to visualize the flame appearance in a furnace. The distribution of the furnace temperature in the BEST conditions was measured with R-type thermocouples. The pollutant emission trend was analyzed when the inlet velocity for fuel jet and oxidizer is 10 m/s to 30 m/s and 10 m/s to 55 m/s, the volumetric mole fraction of oxygen in an oxidizer is 40% to 100%, and the global equivalence ratio is 0.55 to 1.03. The objectives of the present work were to study the flame spectra in normal temperature and pressure (NTP) and the BEST conditions and to determine the global production rate of nitrogen oxides in non-premixed oxy-fuel and oxygen-enriched flames. The visible flame of a non-premixed oxy-methane flame disappeared and temperature deviation in a furnace decreased by 300 degrees C in the BEST conditions. The global production rate of nitrogen oxides ([NOx]/(G)) level decreased while increasing the volumetric mole fraction of oxygen in an oxidizer. The slope of [NOx]/(G) was similar to that of a non-premixed methane-air flame in NTP conditions. The level of [NOx]/(G) was lower in oxygen-enriched flames than in CH4-air flames.