food grain vs oat

food grain and oat both are nouns.

Word	Noun	Adjective	Verb	Adverb
food grain	Yes	No	No	No
oat	Yes	No	No	No

As nouns, oat is a hyponym of food grain; that is, oat is a word with a more specific, narrower meaning than food grain:

food grain: foodstuff prepared from the starchy grains of cereal grasses
oat: seed of the annual grass Avena sativa (spoken of primarily in the plural as `oats')

food grain (noun)	oat (noun)
foodstuff prepared from the starchy grains of cereal grasses	seed of the annual grass Avena sativa (spoken of primarily in the plural as `oats')
	annual grass of Europe and North Africa; grains used as food and fodder (referred to primarily in the plural: `oats')