Other Latin languages (that I am aware of at least) use "si". I know that there is a "si" also in French that is used to answer questions posed in a negative way, but it surprises me the usage of the word "oui" in the other more predominant cases. So, what are the roots of the word "oui"?
I found what you are looking for here:
La forme primitive est oïl, toujours dissyllabe, et formée du latin hoc illud, oui cela (hoc ayant pris le sens de oui : Ne dit ne o ne non, R. de Cambrai 264) ; oïl est donc fait comme nennil, qui représente non illud, non cela. Picard, awi ; Berry, voui ; wallon, awoi, dans lequel Grandgagnage regarde l'a comme prosthétique ; bourguig. vouei ; différents dialectes cités par Grandgagnage ai, âï, oï. On trouve dans les anciens textes, quoique rarement, des formes singulières de ce mot : oal, ouail, ol, odil.
The original form (of oui) is
oïl, and which is compoused of the latin words
hoc illud meaning oui cela (hoc became oui).
oïl is made like the word nennil which represents
non illud meaning non cela.
It is spoken as:
Different dialects mentioned by Grandgagnage:
We can also rarely find in ancient texts very typical forms of oui:
To expand a little ...
First, there was no word for "yes" in Latin. If you wanted to express an affirmation you had to say something explicit, although perhaps terse. E.g. "etiam" ("indeed" or "as always"), "ita est" ("that's it"), "sic est" ("it is thus"), "est" ("it is"), "hoc illic" ("this is it"), etc.
During the Middle Ages the Latin/Romance dialects developed explicit words for "yes" (presumably because other languages that influenced them like Gothic had such a concept). But since there was no single word in Latin they went in different directions, hence the different words. In parts of Spain and Italy the expression "sic est", abbreviated to "sic", became "sí" (and "sim"). In southern France, "hoc illic", abbreviated to "hoc", became "oc". In northern France, "hoc illic" morphed into "oc il'", "oil", and finally "oui"; but they also picked up the "si" thing as well. In Romania, they picked up "da", obviously from Slavic. Sardinian has "eja"; I don't know where that comes from.