This paper reports the possible reaction paths of small silicon clusters Sin(n = 1 4) with the oxygen molecule based on density functional theoretical calculations. It is shown that their potential energy surfaces are very complex, involving spin conserving and inversion. There are no net barriers on any of the proposed reaction paths for the oxidation of Si, Si2, and Si3 clusters, but their reverse reactions are endothermic with high energy barriers. The Si4 cluster cannot react with the ground-state, spin-triplet oxygen molecule because of the potential barrier of its spin inversion. The most favorable and accessible reaction path for Si/Si2/Si3 + O2 may be the one with a high exothermic value since there are no net barriers on the potential energy surfaces. The reactions of silicon clusters with the oxygen molecule are easier than the oxidations of their corresponding suboxides. Our findings are expected to provide valuable information to help understand the growth mechanism of silicon nanowires.