Oxynitridation in nitric oxide of commercial 5 nm and 13 nm oxides is investigated, as well as reoxidation steps. The results are discussed in terms of atomic transport mechanisms and modification of interfacial microstructure. Atomic transport is studied through the use of isotopic tracing and nuclear reactions analyses (NRA): we measured the atomic areal densities (14N, 15N, 16O, 18O) in the films and the concentration depth profiles of the heavy and stable isotopes (15N, 18O) with near surface subnanometric resolution. To scan the evolution of the interfacial microstructure, Pb0 centers concentrations were measured by electron paramagnetic resonance spectroscopy (EPR), after each treatment step. The previously proposed mechanisms for NO oxynitridation of silica are confirmed: i) the NO molecules diffuse via interstitial sites, without reaction with the lattice, before reacting with the dielectric/Si interface; ii) at the external surface region, oxygen exchange is observed due to a step-by-step motion of network oxygen atoms induced by the presence of network defects. During reoxidation in O2 neither loss or observable redistribution of nitrogen is evidenced. For the 13-nm thick film, in the external surface region, the oxygen exchange is seen to be more effective than in the case of silica (or silicon) oxidation: that is O2 is more reactive on NO-nitrided silica than on pure silica. The concentration of Pbo centers decreases after the NO thermal step and increases again after the further reoxidation step.