group O vs blood group

group O and blood group both are nouns.

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

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

group 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 group O include blood type.

group 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