The conversion of complexed nitrogen oxide [Fe(II)EDTA-NO] is the core process in the chemical absorption-biological reduction (CABR) integrated system. Additionally, flue gas of power plants in China contains 3-8% oxygen (O-2), which plays a dominant role in the system. However, previous studies on the reduction of complexed NO were conducted in strictly anaerobic vials. Nitrogen utilization for microorganism growth was not taken into consideration. In this study, the mechanism and conversion pathways of complexed NO were investigated with 0-10% O-2. Results demonstrated that, with O-2, except for being complexed as Fe(II)EDTA-NO, a part of NO was oxidized to NO2. Then, Fe(II)EDTA-NO and NO2 were oxidized to NO2-/NO3- via nitrosification/nitrification or direct chemical reaction, and NO2-/NO3- were reduced to N-2 by complexed NO-reducing bacteria. Besides, 75% of nitrogen was discharged by gas, mainly as N-2. There was 18 and 3% of nitrogen accumulated in biological and liquid phases, respectively.