wild oat grass vs oat

wild oat grass and oat both are nouns.

Word	Noun	Adjective	Verb	Adverb
wild oat grass	Yes	No	No	No
oat	Yes	No	No	No

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

wild oat grass: common in meadows and pastures
oat: annual grass of Europe and North Africa; grains used as food and fodder (referred to primarily in the plural: `oats')

wild oat grass (noun)	oat (noun)
common in meadows and pastures	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')