The entrance channel of the Si+O-2-->SiO+O reaction has been investigated for collinear and perpendicular approach of the silicon atom to the O-2 molecule by ab initio electronic structure calculations using the multireference configuration interaction (MRCI) method and Davidson correction (MRCI+Q). Results show that the reaction can proceed through the ground singlet ((1)A(')) and first triplet ((3)A(')) electronic states at low temperatures. The ground (1)A(') three-dimensional potential energy surface (PES) which correlates the Si(P-3)+O-2(X (3)Sigma(g)(-)) reactants to the SiO(X (1)Sigma(+))+O(D-1) products was computed at the MRCI+Q level of theory using the Woon and Dunning cc-pVTZ basis sets. The reaction was found barrierless and three minima have been characterized on the (1)A(') PES with energy ordering: linear OSiO((1)Sigma(g)(+))