oat vs food grain

oat and food grain both are nouns.

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

As nouns, food grain is a hypernym of oat; that is, food grain is a word with a broader meaning than oat:

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

Other hypernyms of oat include cereal, grain.

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