Oat n. : A well-known cereal grass (Avena sativa), and its edible grain; -- commonly used in the plural and in a collective sense.

Oat n. : A musical pipe made of oat straw.

Example:
Help! Can someone tell me the difference between rollled Oats and Oat bran?

Taxonomy
Oats, groats, oat groats, bushel, cereals, grain, rolled oats, soluble fiber, oat bran, oat flour, yields, acre, protein, wheat, barley, oatmeal, flour, whole oats.

Defination from the web:
oat (Avena sativa) is a species of cereal grain grown for its seed, which is known by the same name (usually in the plural, unlike other grains) ... eaten as porridge, but may also be used in a variety of baked goods, such as oatcakes, oatmeal cookies, and oat bread. ...