office building vs office block

office building and office block both are nouns.

Word	Noun	Adjective	Verb	Adverb
office building	Yes	No	No	No
office block	Yes	No	No	No

As nouns, office building and office block are synonyms defined as:

office building and office block: a building containing offices where work is done

office building (noun)	office block (noun)
a building containing offices where work is done	a building containing offices where work is done