This work studies the parylene-in-oil composite layer as a water protection layer for biomedical applications where adhesion is critical. Compared to solid parylene, the parylene-in-oil film is thermodynamically favorable for preventing water condensation at the interface, subsequent parylene delamination, and underneath material corrosion. To study the effects of silicone oil thicknesses on the composite film, we fabricate test structures and measure both the dry and wet adhesion using a 180° peeling test and an accelerated soak-to-delamination test in saline at 67°C, respectively. Results show that when the oil thickness of the film is between 1-2 µm, the wet adhesion increases a few folds while the dry adhesion remains adequate for implantable medical device applications.