Mycelium-based composite (MBC) consists of the filamentous fungi of mushrooms, mycelium, forming a network with biodegradable agro-waste particles. MBC can be shaped in plastic molds; however, a higher density of mycelium was observed at the MBC surface exposed to the air (MBC/Air) than the MBC contact with plastic mold (MBC/Mold). Consequently, MBC was demolded to obtain uniform growth of mycelium on the substrate. This study investigated the effect of the oxygen transmission rate (OTR) of two different thin film materials, PVC plastic food wrap and stencil paper, on the growth of the mycelium of oyster mushrooms on sawdust. Each thin film was covered between the MBC and polypropylene (PP) mold in configurations of MBC/Film/PP Mold. The OTR of thin films was measured according to ASTM D3985. The results were compared with the OTR of a rigid PVC tube, PET-G, and PP cast, which were used as molds for shaping the MBC in previous literature. It was found that the mycelium was of a higher density in MBC/PVC film/PP and MBC/stencil paper/PP than the top surface of MBC/Air. The OTRs of stencil paper and PVC film were 11,777.78 cc-mm/m²/day and 143.88 cc-mm/m²/day, respectively, which were higher than those of the rigid PVC tube (3 cc-mm/m²/day), PET-G (9.7 cc-mm/m²/day), and PP cast (76 cc-mm/m²/day). Despite the higher OTR found in stencil paper than in PVC film, the mycelium at MBC/PVC film/PP was denser than MBC/stencil paper/PP. This suggested that sufficient oxygen transmission through film contact with the MBC surface was necessary for the mycelium to grow homogeneously. Shaping the complex geometry of MBC can be possible without using the rigid plastic mold, yet only PVC plastic wrap is acceptable. The mechanical properties of MBC will be further investigated.