Oui and si are derived from different roots. Oui is from Latin hoc ille, while si is from Latin sic.
There is actually a si in French that also means yes and comes from the same root as the Spanish and Italian si. However, its usage is much more limited. It’s only used to express disagreement. For example, if I said “Potato chips don’t taste good.” and you wanted to say “Yes, they do!”, then you could use si.
Oui and si are derived from different roots. Oui is from Latin hoc ille, while si is from Latin sic.
There is actually a si in French that also means yes and comes from the same root as the Spanish and Italian si. However, its usage is much more limited. It’s only used to express disagreement. For example, if I said “Potato chips don’t taste good.” and you wanted to say “Yes, they do!”, then you could use si.