The reaction mechanism, rate constants, and product branching ratios for the O(D-1) + NH3 reaction have been studied using ab initio/RRKM methods. The reaction is shown to occur mainly through the insertion mechanism involving the long-lived chemically activated NH2OH* intermediate. The calculated branching ratios of various decomposition products of NH2OH* are in good agreement with recently reported experimental values. The reaction can also proceed through the addition/abstraction mechanism on the first excited-state PES, which is demonstrated to provide some contribution to the NH2 + OH channel and to partially account for the forward scattering of the OH products observed in experiment.