Langue d'oc vs Occitan

Langue d'oc and Occitan both are nouns.

Word	Noun	Adjective	Verb	Adverb
Langue d'oc	Yes	No	No	No
Occitan	Yes	No	No	No

As nouns, Occitan is a hyponym of Langue d'oc; that is, Occitan is a word with a more specific, narrower meaning than Langue d'oc:

Langue d'oc: medieval provincial dialects of French formerly spoken in the south of France
Occitan: the medieval dialects of Langue d'oc (southern France)

Other hyponyms of Langue d'oc include Provencal.

Langue d'oc (noun)	Occitan (noun)
medieval provincial dialects of French formerly spoken in the south of France	the medieval dialects of Langue d'oc (southern France)