painter vs oil painter

painter and oil painter both are nouns.

Word	Noun	Adjective	Verb	Adverb
painter	Yes	No	No	No
oil painter	Yes	No	No	No

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

painter (noun)	oil painter (noun)
large American feline resembling a lion	a painter who uses oil paints
a line that is attached to the bow of a boat and used for tying up (as when docking or towing)
an artist who paints
a worker who is employed to cover objects with paint