Langues d'oïl
They share many common linguistic features.  One of them is that Oïl (pronounced [wil], or [wi] as in modern French oui), was the ancestor of the word for yes that is used in all of the Oïl languages.  Oc is the word used in territories to the south and so the Occitan language is also called langue d'oc (literal English translation: "language of yes").