This study evaluated the mercury sorption by activated carbon (AC) under an O-2/CO2 atmosphere in a fixed-bed reactor. Effects of the oxygen concentration on the mercury sorption efficiency under both air and oxy-fuel atmospheres were explored. The kinetic studies were also used to predict the mercury sorption process by the pseudo-first-order model and the intraparticle diffusion model in this work. The experimental results indicated that the mercury sorption capacity of AC increased with the increased oxygen concentration under both air and oxy-fuel atmospheres. Oxygen might increase the oxidation of mercury by the Mars Maessen way. A high CO2 concentration promoted AC to generate more active sites under an oxy-fuel atmosphere. Besides, the results of the kinetic analysis illustrated that the pseudo-first-order model showed better agreement with the experimental data compared to the intraparticle diffusion model. These experimental and theoretical results in this work are helpful in mercury capture under an oxy-fuel atmosphere.