Hemicellulose and lignin act as physical barriers impeding the hydrolysis of lignocellulosic biomass by cellulases. To develop a process which could simultaneously remove most hemicellulose and lignin in biomass, in this study, oxygen-assisted ethanol organosolv pretreatment (O-2-EOP) of sugarcane bagasse (SCB) was carried out. The effects of temperature, time, oxygen pressure and ethanol concentration on the pH of the hydrolysate, the solubilization and the chemical composition of SCB were investigated. Compared with autocatalytic EOP and acid-catalyzed EOP, O-2-EOP could remove most xylan and lignin from SCB at much milder conditions. At optimized conditions (40/60 ethanol/water (v/v), 1.5 MPa O-2, 160 A degrees C and 80 min), 82.9% xylan and 83.3% lignin were removed. Glucan content in the residue reached 86.3%, much higher than results obtained by autocatalytic EOP and acid-catalyzed EOP. In subsequent enzymatic hydrolysis (1% solid loading, 4.96 mg protein/g cellulose, 72 h), O-2-EOP pretreated SCB produced 3.97 and 1.90 times more glucose than untreated material and N-2-EOP pretreated SCB, respectively. O-2-EOP also produced abundant organic acids, total amounts of formic acid and acetic acid in the hemicellulose hydrolysate reached 5.42 g/L. In conclusion, O-2-assisted EOP was an effective process for the production of pulp with high cellulose purity and good accessibility to cellulases.