oppressiveness vs onerousness

oppressiveness and onerousness both are nouns.

Word	Noun	Adjective	Verb	Adverb
oppressiveness	Yes	No	No	No
onerousness	Yes	No	No	No

As nouns, oppressiveness and onerousness are synonyms defined as:

Other synonyms of oppressiveness include burdensomeness, heaviness.

oppressiveness (noun)	onerousness (noun)
unwelcome burdensome difficulty	unwelcome burdensome difficulty
a feeling of being oppressed