We report on the change of chemistry involved by molecular nitrogen in the deposition process of pp-HMDSO thin films with an AP-DBD, using HMDSO as chemical precursor. By modifying the composition of the main gas from pure argon to pure nitrogen, thin films composition varied from SiOC:H to SiOCN:H. A small amount of nitrogen favours polymer chain propagation, by consuming species responsible for chain termination and playing a role in propagation phase. Higher nitrogen content leads to more cross-linked coatings. The use of optical emission spectroscopy together with FT-IR and XPS is shown to be relevant to study such processes.