open house vs party

open house and party both are nouns.

Word	Noun	Adjective	Verb	Adverb
open house	Yes	No	No	No
party	Yes	No	Yes	No

As nouns, party is a hypernym of open house; that is, party is a word with a broader meaning than open house:

open house (noun)	party (noun)
an informal party of people with hospitality for all comers	an occasion on which people can assemble for social interaction and entertainment
	a gathering of people for pleasure
	an organization to gain political power
	a band of people associated temporarily in some activity
	a person involved in legal proceedings

open house (verb)	party (verb)
	have or participate in a party