The O(D-1 and P-3)+SiH4 reactions have been studied using ab initio/Rice-Ramsperger-Kassel-Marcus calculations to investigate possible formation mechanisms of various products in combustion and chemical vapor deposition processes. The relative branching ratios for various products formed through the O(D-1)+SiH4 reaction involving the long-lived H3SiOH intermediate are calculated as 55.5% for the H2SiO/HSiOH+2H channel, 28.4% for the SiO+2H(2) channel, 9.9% for the OH+SiH3 channel, 3.2% for the H2O+SiH2 channel, and 3.0% for the HSiO/SiOH+H-2+H channel. These results significantly differ from those obtained in experiment, implying that the O(D-1)+SiH4 reaction can take place through a mechanism other than the insertion mechanism. While the O(P-3)+SiH4 reaction takes place by the abstraction mechanism, the O(D-1)+SiH4 reaction can occur through both insertion and addition/abstraction mechanisms. The addition/abstraction mechanism occurring on the first excited potential energy surface is demonstrated to provide a significant contribution to the reaction products and to account for the forward scattering of the OH products observed in experiment. Finally, heats of formation for various species involving Si atom are computed employing the Gaussian 3 theory.