preen gland vs oil gland

preen gland and oil gland both are nouns.

Word	Noun	Adjective	Verb	Adverb
preen gland	Yes	No	No	No
oil gland	Yes	No	No	No

As nouns, oil gland is a hypernym of preen gland; that is, oil gland is a word with a broader meaning than preen gland:

preen gland: oil-secreting gland situated at the base of the tail in most birds
oil gland: a gland that secretes oil

preen gland (noun)	oil gland (noun)
oil-secreting gland situated at the base of the tail in most birds	a gland that secretes oil