Partial oxidative pyrolysis of red oak was studied in a laboratory-scale, continuous fluidized bed reactor. The concentration of oxygen in sweep gas was between 0.525% and 8.40% (v/v), corresponding to equivalence ratios of 0.034-0.539. The influence of oxygen on the distribution and properties of pyrolysis products was investigated using various analytical methods, including GC-MS/FID, IC, HPLC and FT-IR. It was found that total bio-oil yield was not affected by varying oxygen concentration (61.2-64.8 g/100 g bio-mass) although the water content of the bio-oil increased. The elemental carbon content of bio-oil and biochar decreased with increasing oxygen concentration, being converted to non-condensable gases mainly CO and CO2. As expected, the presence of high concentration of oxygen in the sweep gas stream had detrimental effects on the pyrolysis products, including increase in water and oxygen content. However, the results also revealed that a small amount of oxygen in the sweep gas (0.525-1.05%, v/v) improved the yield of hydrolyzable sugars and total yield of phenolic monomers. The bio-oil produced under oxidative pyrolysis also contained less acid and pyrolytic lignin. Oxygen increased the surface area of biochar. The results of this study suggest that carefully controlled partial oxidative pyrolysis can improve the quality of bio-oil from fast pyrolysis, increasing the content of sugars and phenolic monomers. (C) 2014 Elsevier Ltd. All rights reserved.