type O vs blood group

type O and blood group both are nouns.

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

As nouns, blood group is a hypernym of type O; that is, blood group is a word with a broader meaning than type O:

type O: the blood group whose red cells carry neither the A nor B antigens
blood group: human blood cells (usually just the red blood cells) that have the same antigens

Other hypernyms of type O include blood type.

type O (noun)	blood group (noun)
the blood group whose red cells carry neither the A nor B antigens	human blood cells (usually just the red blood cells) that have the same antigens