Polylactide (PLA)/polyphenylene oxide (PPO) composites were prepared in a wide range of compositions by mixing solutions of PLA and PPO in chloroform and used to obtain dense thin films. It was found that the aggregates formed in solutions of PLA/PPO blends were more compact compared with those existing in solutions of individual macromolecules. The structural features of PLA/PPO composite films were studied using wide-angle X-ray diffraction analysis, scanning electron microscopy, thermogravimetric analysis, and estimated in comparison with the properties of pristine PPO and PLA films. Gas transport properties were determined by measuring H2, CO2, O2, and N2 permeability. Preparation of the PLA/PPO composites improved the properties of the initial polymers: gas permeability of PLA was increased, and the PPO selectivity was enhanced. It was revealed that composite PLA/PPO (30) exhibited the optimal transport properties. The molecular dynamics simulations of the PLA/PPO (30) blend were performed for a more detailed study of the effect of PPO introduced into the PLA matrix on the structure and properties of the composites. The obtained PLA/PPO composites were thermally stable and exhibited good mechanical properties: elevated strength (43–50 MPa) in combination with high rigidity (1600–1950 MPa).