Old Testament vs Book of Obadiah

Old Testament and Book of Obadiah both are nouns.

Word	Noun	Adjective	Verb	Adverb
Old Testament	Yes	No	No	No
Book of Obadiah	Yes	No	No	No

Old Testament (noun)	Book of Obadiah (noun)
the collection of books comprising the sacred scripture of the Hebrews and recording their history as the chosen people; the first half of the Christian Bible	an Old Testament book telling Obadiah's prophecies; the shortest book in the Christian Bible