An oil seal on a motorcycle is a sealing component designed to prevent oil leaks and keep lubrication fluids contained within engine or transmission parts. It provides a tight seal between moving and stationary parts, such as shafts and housings, to protect against oil leakage and contamination. Made from durable materials like rubber or synthetic compounds, the oil seal ensures proper function and longevity of the engine or transmission by maintaining effective lubrication and preventing dirt and moisture from entering the system.