oat vs wild oat grass

oat and wild oat grass both are nouns.

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

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

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

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