door vs open door

door and open door both are nouns.

Word	Noun	Adjective	Verb	Adverb
door	Yes	No	No	No
open door	Yes	No	No	No

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

door (noun)	open door (noun)
a swinging or sliding barrier that will close the entrance to a room or building or vehicle	freedom of access
the entrance (the space in a wall) through which you enter or leave a room or building; the space that a door can close	the policy of granting equal trade opportunities to all countries
a room that is entered via a door
a structure where people live or work (usually ordered along a street or road)
anything providing a means of access (or escape)