Research on silicon oxide thin films developed as gas-barrier protection for polymer-based components is reviewed, with attention paid to the relations between (i) coating defects, cohesive strength and internal stress state, and (ii) interfacial interactions and related adhesion to the substrate. The deposition process of the oxide from a vapor or a plasma phase leads in both cases to the formation of covalent bonds between the two materials, with high adhesion levels. The oxide coating contains nanoscopic defects and microscopic flaws, and their respective effect on the barrier performance and mechanical resistance of the coating is analyzed. Potential improvements are discussed, including the control of internal stresses in the coating during deposition. Controlled levels of compressive internal stresses in the coating are beneficial to both the barrier performance and the mechanical reliability of the coated polymer. An optimal coating thickness, with low oxygen permeation and high cohesive strength, is determined from experimental and theoretical analyses of the failure mechanisms of the coating under mechanical load. These investigations are found relevant to tailor the interactions and stress state in the interfacial region, in order to improve the reliability of the coating/substrate assembly.