The reaction of Co+ with OCS on both triplet and quintet surfaces has been investigated at the B3LYP level of theory of density functional theory with the standard 6-311 + G(d) basis set. The object of this investigation was the elucidation of the reaction mechanism. The calculated results indicate that both the C-S and C-O bond activations proceed via an insertion-elimination mechanism. Although the direct sulfur and oxygen atom abstractions by Co+ might contribute to the formations of CoS+ and CoO+, the tight transition state structures were not found in the present calculations. Intersystem crossing between the triplet and quintet surfaces may occur along both the C-S and C-O bond activation branches. The ground states of CoS+ and CoO+ were found to be quintets, whereas CoCO+ and CoCS+ have triplet ground states. The C-S bond activation is energetically much more favorable than the C-O bond activation. All theoretical results are in line with early experiments.