"consisting of oats or oatmeal," late 14c., oten, from oat + -en (2). Also "made of stem of the straw of oats," as shepherd's pipes were in poetry.
"Western film" (featuring horse-riding cowboys and Indians), 1946, American English, from oat, as the typical food of horses. Oats opera (on the model of soap opera) is by 1937 in U.S. slang.
"the coarsely ground meal of oats," late 14c., ote-mele, from oat + Middle English mele (see meal (n.2)).