In this paper, which is the first of a series devoted to some aspects of the title reaction, we present the theoretical results concerning the topology of the lowest potential energy surface of triplet multiplicity. Results at the ab initio CASPT2 level with a complete active space self-consistent field (CASSCF) reference wave function built on a D95 Dunning basis set and involving 10 electrons in 10 orbitals are reported. This triplet surface is expected to play an important role in this reaction since experimental results show HCN(X-1 Sigma(+)) as the main product, and this product can only be obtained via an HCNO((3)A) hypersurface. We find that the channel leading to the formation of HCN(X-1 Sigma(+)) + O(P-3) is the most likely product channel, in agreement with available experimental findings. To a lesser extent, we can also observe the formation of the product channels HCO(X(2)A')+N(S-4), NCO((XII)-I-2)+H(S-2), and CO(X-1 Sigma(+))+NH(X-3 Sigma(-)). Conversely, the formation of CN(X-2 Sigma(+))+OH((XII)-I-2) is highly unlikely because of the existence of high potential energy barriers along this reaction path.