The effects of different process parameters (alkali dosage, hydrogen peroxide dosage, temperature, reaction time, and oxygen pressure) on oxalate formation during hydrogen peroxide-reinforced oxygen delignification (OP) of Eucalyptus kraft pulp were investigated in the present study. The relationships between oxalate formation and the dosages of both alkali and peroxide were found to be almost linear. Oxalate formation could be divided into “fast” and “slow” periods, depending on the reaction time. The best selectivity for the OP process was achieved using an alkali dosage of 2.0%, a hydrogen peroxide dosage of 2.0%, a temperature of 100°C, an oxygen pressure of 0.6MPa and a reaction time of 60min. Oxalate formation was also, to some extent, reduced under these conditions. These results will be very helpful in optimizing the OP process, controlling oxalate formation and improving product quality.