OIS vs government agency

OIS and government agency both are nouns.

Word	Noun	Adjective	Verb	Adverb
OIS	Yes	No	No	No
government agency	Yes	No	No	No

As nouns, government agency is a hypernym of OIS; that is, government agency is a word with a broader meaning than OIS:

OIS: agency that oversees the intelligence relationships of the Treasury's offices and bureaus and provides a link between the Intelligence Community and officials responsible for international economic policy
government agency: an administrative unit of government

Other hypernyms of OIS include agency, authority, bureau, federal agency, office.

OIS (noun)	government agency (noun)
agency that oversees the intelligence relationships of the Treasury's offices and bureaus and provides a link between the Intelligence Community and officials responsible for international economic policy	an administrative unit of government