We performed a detailed study of the NH + O-2 potential energy surface by means of a number of multireference (CASSCF, MC-QDPT2, MR-AQCC, MR-CISD(18;13)+Q with 6-311+G(d,p), and aug-cc-pVTZ basis sets) and composite (G3B3, G3MP2B3, CBS-QB3, W1U) methods. Parent nitroso oxide, HNOO, was found to be the key intermediate of this process. In its ground state, (1)A', HNOO exists in two conformations, where the cis form is 8.1 - 10.9 kJ.mol(-1) more stable than the trans-nitroso oxide. The mechanism of nitrene oxidation by dioxygen may be represented as a set of various transformations of vibrationally excited HNOO, namely, decomposition into NO and OH radical pair, O-O dissociation reaction, and a number of thermal deactivation processes. We localized all stationary points of these transformations on both the singlet and the triplet reaction PES. The energies of reactants, products, and transition states were calculated at the RI-MR-CISD(18;13)+Q/aug-cc-pVTZ level of theory; the vibrational analysis of these species was done by means of CASSCF(18;13)/6-311+G(d,p). Apparent rate constants of the NH + O-2 reaction were calculated using RRKM theory. The total rate constant k(total) corresponds well to available experimental data. The temperature dependence of k(total) is rather nontrivial and consists of three quasi-linear intervals. At low temperatures (up to room temperature) the slope of log(k(total)) vs 1/T is negative due to prevailing stabilization of HNOO. The rate-determining channel of the "NH + O-2" reaction in the medium-temperature interval (up to similar to 1000 K) was found to be formation of the NO + OH radical pair via H transfer to the terminal oxygen atom. This reaction is accelerated by a factor of 4.2 (214 K) and 1.2 (2500 K) due to tunnel effect. The distinctive feature of the NH + O-2 high-temperature chemistry is the increase of the effective activation energy due to prevailing dissociation of the HNOO peroxide bond.