Old French had uns/unes (both nominative singular and accusative plural, of course), but they were mostly used for a singular set of plural elements, like "unes armes" meaning weapons, in the sense of enough weapons to arm one soldier, or unes lèvres (lips), unes grans narines (big nostrils).
Dē could already be used with partitive meaning in Latin ("Probet autem seipsum homo, et sic de pane illo edat, et de calice bibat" from the Vulgate (I Corinthian 11)) and this seems to have remained a low frequency option (it's cross linguistically common for ablative markers to be useable with a partitive meaning) until Late Old French, when fully grammaticalised partitive articles start to emerge from this occasional partitive usage of de. Once de+article has become established as a partitive article, it quickly displaces un(e)s as the plural indefinite article, since there's a heavy semantic overlap between plural indefinites and plural partitives.
The uses of de + definite article in OF seem roughly similar to those of de + demonstrative determiner in modern French, with the meaning of "some of a previously established concrete thing", almost always in direct objects of verbs of eating or drinking: "le gastel et le vin lor baille, // .i. fromage lor pere et taille. // Cil mangierent qui fain avoient, // et del vin volantiers bevoient" (modern: Il leur donne du pain et du vin, leur prépare et leur coupe du fromage. Ceux qui avaient faim mangèrent, et burent volontier de ce vin)
Carlier & Lamiroy 2014 (The grammaticalization of the prepositional partitive in Romance) catalogue several parallel uses of de as partitive marker in Old Spanish:
(El Cid) Y fallaron un vergel con una muy limpia font, [...] Dadnos agora del agua, ¡sí vos vala el Criador!”. Estonces con un sonbrero que tiene Félez Muñoz (mucho nuevo era e fresco, que de Valencia sacó), cogió del agua con él e a sus primas la dio.
But in Spanish this never led to further grammaticalisation of the type we saw in French or in Italian.
