Oxy-fuel combustion is a recognized technology for capturing carbon dioxide (CO2) from the power and industrial processes. Its retrofitting potential is an important characteristic for both the re-vamping of existing conventional boilers and the design of capture ready plants. In this study a light oil fired Central European Norm boiler has been retrofitted to oxy-fuel mode by simply replacing the burner blower by a premixing chamber for O2 and CO2 for simulating dry flue gas recirculation. Although the transition from air to oxy-fuel did not show major issues, the heat transfer and thermal efficiency of the boiler were affected. The thermal efficiency was found to monotonically increase with increasing O2 concentration in the oxidizer, and was equal to that of air at 32-35% O2 depending on operating equivalence ratio. Higher heat fluxes in the radiant section of the burner were also measured for all oxy-fuel cases investigated as compared to that of air. To further investigate the differences in radiative heat transfer mechanisms implicated in the boiler, the study was pursued at laboratory scale and laminar conditions to analyse the influence of O2 and CO2 on both the radiative properties and soot formation. The measurements have highlighted the global effects of the oxidizer composition, where the flame lengths were shorter, the radiant fraction and peak radiative heat flux increase as the O2 concentration increased. The radiant fraction for a 35 % O2 mixture in CO2 combustion was found to be equal to that of an air flame despite a difference in adiabatic flame temperature. Both the O2 concentration and the replacement of N2 by CO2 were shown to have an influence on soot formation. The peak soot volume fraction increased with O2 concentration until 70% O2 in O2 - CO2 oxidizers, but was found to be lower than in oxygen enriched air flames at similar Reynolds number conditions reported in the literature, supporting the idea that the increase in radiative heat flux in O2 - CO2 systems is more dominated by temperature than soot formation.