OB vs medical specialty

OB and medical specialty both are nouns.

Word	Noun	Adjective	Verb	Adverb
OB	Yes	No	No	No
medical specialty	Yes	No	No	No

As nouns, medical specialty is a hypernym of OB; that is, medical specialty is a word with a broader meaning than OB:

OB: the branch of medicine dealing with childbirth and care of the mother
medical specialty: the branches of medical science that deal with nonsurgical techniques

Other hypernyms of OB include medicine.

OB (noun)	medical specialty (noun)
the branch of medicine dealing with childbirth and care of the mother	the branches of medical science that deal with nonsurgical techniques