An oil gland is a sebaceous (or sebiferous) gland that secretes sebaceous matter. Oil glands are part of the body's integumentary system and serve to protect the body against germs. They are mainly situated in the corium or true skin. True oil glands secrete matter through a duct. Many creatures, such as fish, e.g., have oil glands. In mammals they serve as a protectant and lubricant for hair and skin. In birds it is located at the base of the tail, which also supplies oil that is spread upon the feathers during preening.
Ductless glands situated internally are not true oil glands, however, fishermen will often cleanse bottom-feeding fish such as carp by removing an internal organ (which is said to be "the oil gland"). Ductless internal organs are called "glands," as well. Having no ducts, they tend to store substances which give a disagreeable taste to a cooked fish. Removing "the oil gland" from a carp prior to cooking removes some of the accumulated substances, some of which may also be toxic or harmful to a person who eats fish.