The O-2 and O-2-N-2 ([N-2] < 15%) post-discharge microwave plasma modifications of high-density polyethylene (HDPE) and hexatriacontane (HTC) surfaces have been studied as a function of the distance from the discharge and the gas composition. They are compared in terms of the in-situ XPS O/C ratios and C1s components, and the ex-situ ToF-SIMS O-/CH- ratios and relative intensities of series of peaks. The results on the effect of the distance from the discharge showed a clear correlation between the in-situ XPS results and the O-2 post-discharge modeling, exhibiting the double role of oxygen atoms: functionalization initialization by creating radicals (which react with molecular oxygen) and degradation due to the energy released by the oxygen atom recombination process. Specific in-situ XPS and ex-situ ToF-SIMS signatures of this in-situ degradation related to the oxygen atom recombination process were exhibited. When N-2 was introduced in the plasma gas, the in-situ XPS results and the ex-situ ToF-SIMS results were very different. The in-situ functionalization decreased as a function of the N-2 addition and the ex-situ functionalization exhibited a high maximum fdr the 5% N-2-95% O-2 post-discharge plasma and then decreased. Despite the absence of a complete O-2-N-2 post-discharge modeling, it can be assumed that there is also a maximum of the oxygen atom content for the 5% N-2-95% O-2 postdischarge. Thanks to the in-situ XPS and ex-situ ToF-SIMS specific signatures, it was also shown that this maximum corresponds to a low in-situ degradation effect. Nitrogen introduction could influence the role of oxygen atoms in such a way that there is a decrease in oxygen atom recombination processes (thus in degradation) for small N-2 addition and even a decrease in oxygen functionalization initialization for further N-2 incorporation in the plasma gas. No nitrogen was observed in the in-situ XPS results, whereas some ex-situ ToF-SIMS nitrogen-containing ions were observed for the O-2 and O-2-N-2 post-discharge. However, their relative intensities followed the variation in oxidation and not the variation in N-2 concentration in the plasma gas. They could be related to a post-treatment functionalization effect. Differences observed between HDPE and HTC are explained in terms of their structural differences (desorption of low molecular weight oxygen-containing fragments for HTC).