oilman vs skilled workman

oilman and skilled workman both are nouns.

Word	Noun	Adjective	Verb	Adverb
oilman	Yes	No	No	No
skilled workman	Yes	No	No	No

As nouns, skilled workman is a hypernym of oilman; that is, skilled workman is a word with a broader meaning than oilman:

Other hypernyms of oilman include skilled worker, trained worker.

oilman (noun)	skilled workman (noun)
a worker who produces or sells petroleum	a worker who has acquired special skills
a person who owns or operates oil wells