Old French (franceis, françois, romanz; Modern French ancien français) was the Romance dialect continuum spoken in territories that span roughly the northern half of modern France and parts of modern Belgium and Switzerland from the 9th century to the 14th century. It was then known as the langue d'oïl (oïl language) to distinguish it from the langue d'oc (Occitan language, also then called Provençal), whose territory bordered that of Old French to the south. The Norman dialect was also spread to England, Ireland, the Kingdom of Sicily and the Principality of Antioch in the Levant.
Gaulish, maybe the only survivor of the continental Celtic languages in Roman times, slowly became extinct during the long centuries of Roman dominion. Only several dozen words (perhaps 200, if we add Gaulish etymology) survive in modern French, for example chêne, ‘oak tree’ and charrue ‘plough'; Delamarre (2003, pp. 389–90) lists 167.
Despite attempts to explain some phonetic changes being caused by a Gaulish substrate, only one of them is sure, because this fact is clearly attested in the Gaulish language epigraphy, e.g. : on the potteries of la Graufesenque (1st century AD), there is the Latin word (from Greek) written paraxsid-i instead of paropsid-es. The spellings /ps/ and /pt/ are confused with /xs/ and /xt/, e.g. : Latin capsa > *kaxsa > caisse (compare Italian cassa) or captīvus > *kaxtivus > OF chaitif (Modern French chétif, compare Irish cacht 'servant' ≠ Italian cattiv-ità, Spanish cautivo). This phonetic evolution is parallel to the shift of the Latin group /kt/ in Old French (Latin factum > fait, Italian fatto, Spanish hecho; or lactem* > lait, Italian latte, Spanish leche).
Old French began when the Roman Empire conquered Gaul during the campaigns of Julius Caesar, which were almost complete by 51 BC. The Romans introduced Latin to southern France by 120 BC when it came under Roman occupation.
Beginning with Plautus's time (254–184 BC), the phonological structure of classical Latin underwent change, which would eventually yield vulgar Latin, the common spoken language of the western Roman empire. This latter form differed strongly from its classical counterpart in phonology; it was the ancestor of the Romance languages, including Old French. Some Gaulish words influenced Vulgar Latin and, through this, other Romance languages. For example, classical Latin equus was replaced in common parlance by vulgar Latin caballus, derived from Gaulish caballos (Delamare 2003 p. 96), giving Modern French cheval, Catalan cavall, Occitan caval (chaval), Italian cavallo, Portuguese cavalo, Spanish caballo, Romanian cal, and (borrowed from Anglo-Norman) English cavalry and chivalry.