Oc comes from Latin hoc, neutral form of pronoun which means “that’s it”. No specific word was available at the time to say yes.
From this comes the name of the Languedoc region, namely land of Oc language, situated around the city of Toulouse, South of France. The term Occitany appeared in the Middle Age comes also from Latin Occitania, same construction than the existing Aquitania (the region around Bordeaux) at Roman Empire times.
Today, in the Occitan language, yes is always “oc”, but the final letter c is not pronounced.
All along the Middle Age, in the North of France, two ways existed to say yes: o and oil. That word derives also from Latin hoc and ille (this).
The prononciation was not [oï] but o-il. As the prononciation of the letter o moved to [ou], oil was said [ou-il], then the final letter l disappeared (like the c of oc in the South) to end with [ou-i] and finally: oui! As it is always the case in French, language of France, from the name of the tribe: the Franks.
A complete review of Occitan literature translated into English has been published recently and is available in our cottage.