expulsion vs ouster

expulsion and ouster both are nouns.

Word	Noun	Adjective	Verb	Adverb
expulsion	Yes	No	No	No
ouster	Yes	No	No	No

As nouns, ouster is a hyponym of expulsion; that is, ouster is a word with a more specific, narrower meaning than expulsion:

Other hyponyms of expulsion include defenestration, deportation, ostracism, barring, blackball, ousting.

expulsion (noun)	ouster (noun)
the act of expelling or projecting or ejecting	the act of ejecting someone or forcing them out
the act of forcing out someone or something	a wrongful dispossession
squeezing out by applying pressure	a person who ousts or supplants someone else