In package manufacturing, in addition to the tightness and the strength of the created seam joints, the other properties of plastic materials are also sought to be preserved. This aspect is particularly emphasized in the case of the barrier properties of food packaging made of multilayer plastic films. The aim of the study was to investigate how joining plastic layers together by sealing affects the oxygen barrier properties of the resulting material pair. In addition, the objective was to evolve the seal property measurement methods. Sealed samples were prepared by joining plastic film materials of different grades in combinations often used in packaging production, such as single material container-like packages. The samples included a few examples of oil-based plastics and bioplastics, of which the latter are becoming more important due to the appreciation of sustainability. The experimental part comprised a series of seal strength and oxygen transmission rate tests. With the designed custom sample holder, the application of the analysis method can be expanded to samples with moderate deformations and to samples cut from ready-made packages, which are often of interest in practical packaging research. The study enhanced the understanding of the effect of sealing parameter adjustment on the seal strength and oxygen barrier properties which are prerequisites for packaging performance. There are significant differences in the processing of different film materials and the effect of adjustment parameters on the properties of the resulting sealed seam can only be properly assessed with a combination of measurement methods selected based on the future application.