In Roman Gaul everyone spoke Latin, of course. Now, Classical Latin had no word simply for "yes"; to give a positive answer to a question you had to use a whole sentence, e.g. the equivalent of 'I am', 'It is', 'He does' and so on. At some point in Late Latin people found this tiresome and took to replying to questions with 'Hoc ille' - 'That's it'. Over the centuries, in Southern France the tail-end of this phrase was dropped and it became 'Oc', while the Northerners stressed the opposite end of the phrase and it became 'Oil'.