dictator vs oppressor

dictator and oppressor both are nouns.

Word	Noun	Adjective	Verb	Adverb
dictator	Yes	No	No	No
oppressor	Yes	No	No	No

As nouns, oppressor is a hypernym of dictator; that is, oppressor is a word with a broader meaning than dictator:

dictator (noun)	oppressor (noun)
a ruler who is unconstrained by law	a person of authority who subjects others to undue pressures
a person who behaves in a tyrannical manner
a speaker who dictates to a secretary or a recording machine