Oxy-fuel combustion, which is a promising technology for the abatement of carbon dioxide emissions, can be applied in circulating fluidized-bed (CFB) power plants. In this study, the effects of operational conditions on the progress of oxy-fuel CFB combustion were investigated by means of a mathematical model for CFB oxy-fuel combustion together with experimental data from a 4 MWth oxy-fuel CFB, currently representing the largest oxy-fuel CFB combustion experiments in the literature. Modeled in-furnace profiles for carbon monoxide (CO) and oxygen (ox) were compared to the corresponding measurements, yielding a general good agreement for both air- and oxy-fuel-fired conditions. The developed model was also used to investigate the effects of varying the inlet O-2 concentration over a wider range than that applied in the experiments. The experimental results show that, for an equivalent inlet O-2 concentration, the peak CO concentration is higher under oxy-fuel-fired conditions than under air-fired conditions. The model result shows that a higher inlet O-2 concentration generates combustion of greater intensity up through the furnace with a lower level of CO at the furnace exit.