Oxy-combustion of biomass is a potentially attractive and yet largely unexplored technology facilitating the negative generation of CO2. In this paper, numerical simulations are conducted to investigate the transient combustion process of a single biomass particle in O-2/N-2 and O-2/CO2 atmospheres, and the results are validated against the existing experimental data. Oxygen concentration varies from 27% to 100% in the investigated gaseous atmospheres. The spatiotemporal evolutions of the gas-phase temperature and species concentration fields are explored to further understand the transient combustion characteristics of biomass particles in oxygenated atmospheres. The results show considerably different burning behaviors under carbon dioxide- and nitrogen-containing atmospheres. Simultaneous and sequential combustion of the volatiles and char are distinguished from the numerical simulations. Further, NOx and SOx emissions are predicted on the basis of the validated numerical combustion model. A qualitative analysis is then performed to investigate the influences of oxygen concentration and carbon dioxide atmosphere upon the pollutant emissions. It is shown that CO2 has a significant inhibitory effect on NOx formation, while it promotes SO2 emissions. As oxygen concentration increases, the NO and SO2 emission rates decrease under both types of gas atmospheres. Nonetheless, the overall NOx and SOx emissions feature different trends.