Oil/water separation media, such as membrane, mesh and fabric, have been heavily researched in recent years. However, most separation materials need complex modification and suffer from high costs. In this study, we piled up micron-scaled oyster shell particles (OSPs) to form a layer for separating oil/water immiscible mixtures, avoiding the complex fabrication process and yielding high separation efficiency. The prepared OSP layer exhibits underwater hydrophilicity resulting from the hydrophilicity of its main component CaCO3 and the multilevel structure of its surface, which allows only water to pass through and prevents oil from permeating. It is worth mentioning that the separation efficiencies of the OSP layer for all tested oil/water immiscible mixtures are all above 99.9% and the underwater contact angles for all investigated oils are all above 130°. We believe that this eco-friendly and profitable packing layer has a great future and will be applied in a large scale.