This study focuses on the synergistic properties of three types of coal when co-fired with oat straw at different blending ratios. The results demonstrated non-additive interaction between oat straw and coal samples. The catalytic effect of oat straw ash and the non-catalytic effect of its organic constituents on these coal samples were isolated and analysed to measure their contribution to the confirmed synergistic effects during co-firing. The results showed a level of synergy suppression between catalytic and non-catalytic effects due to the overlapping function of the catalytic AAEMs and the radical propagation promoted by organic content of biomass. A novel index, i.e., synergy combination efficiency, was therefore proposed and used to quantify the level of synergistic promotion or synergistic inhibition occurring during the co-firing of these fuel blends. It was found that at a blending ratio of 30?wt% oat straw, the Guizhou coal achieved a synergy factor (S.F) of 1.50, with non-catalytic and catalytic synergy contributing 69.1% and 30.9% respectively. This coal blend showed the highest synergistic promotion with a combined efficiency of 194%, which demonstrated the potential of the use of co-firing synergy to improve combustion performance of poor quality coals.