group O vs O

group O and O both are nouns.

Word	Noun	Adjective	Verb	Adverb
group O	Yes	No	No	No
O	Yes	No	No	No

As nouns, group O and O are synonyms defined as:

group O and O: the blood group whose red cells carry neither the A nor B antigens

Other synonyms of group O include type O.

group O (noun)	O (noun)
the blood group whose red cells carry neither the A nor B antigens	the blood group whose red cells carry neither the A nor B antigens
	the 15th letter of the Roman alphabet
	a nonmetallic bivalent element that is normally a colorless odorless tasteless nonflammable diatomic gas; constitutes 21 percent of the atmosphere by volume; the most abundant element in the earth's crust