The oxygen transmission rate of 48 nm thick SiO2 coatings on a poly(ethylene terephthalate) (PET) substrate was reduced by a factor of three after applying an 8 nm thick layer of oligomerized gammaaminopropyltriethoxysilane (γ-APS). Increasing the thickness of the γ-APS layer up to 600 nm did not further reduce the oxygen transmission rate of the composite material. The influence of the silane treatment on the barrier performance of the SiO2 coating was investigated using a combination of surface spectroscopy analysis, reactive ion etching and optical and electron microscopy. With the permeation modelling of macro-defect and nano-defect population, it was found that permeation through macro-defects in untreated SiO2 represents approximately 2/3 of the film permeation. Moreover, these analyses confirmed the defect-healing effect of the amino-silane on the SiO2 barrier coating, with a two-fold reduction of the macro-defect density in the oxide, and a two-fold reduction of its oxygen permeability.