Oilskin can mean:

These days, oilskins or oilies means the foul-weather gear worn by sailors, made of modern synthetic and often quite advanced fabrics. (At sea, wet can come from sea spray as well as from rain.)

Oilskin trousers are very high-cut for a large overlap with the jacket and prevent water entering through the join. Often in moderate weather, however, only the trousers are worn (as in the right of the photo) and their high cut then keeps wind and water off the lower part of the torso. It has shoulder straps to hold the trousers up. Straps around the bottom of the trouser legs let them be tightened around seaboots, providing a semi-watertight join. This does not let them be used like fishing waders, but a wave sweeping briefly across the deck will generally not penetrate. All but the cheapest oilskin trousers will be reinforced across the seat and the knees

Oilskin jackets are similar in many ways to waterproofs used for walking. The most visible difference is that they usually have a much higher collar to keep out spray, and in many cases to warm the ears or the whole head. A fold-away hood will be provided, almost always in a high-visibility colour since the head will be the only part showing above the water if the sailor is lost overboard. It often has retroreflective patches on the shoulders for the same reason. Its tails are very long to keep water off the legs.