office block vs office building

office block and office building both are nouns.

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

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

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

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