This category tracks Catalan nouns and adjectives which unusually carry an ending -o where -e would normally be expected.

Early in the evolution of Gallo-Romance, unstressed final vowels (other than /a/, which will be ignored for what follows) were nearly always lost in polysyllabic words unless needed to support a preceding consonant cluster. In the latter case, all vowels yielded /ə/ in French and /e/ in Occitan, no matter what they had been in Latin. Notably, Franco-Provençal retained a distinction between original front and back vowels. Consider the following examples:

Catalan generally shows the French– and Occitan-type merger (cf. mare, quatre). Nevertheless, a number of seemingly inherited masculine forms show final -o, generally alongside variants showing -e instead. In some cases one variant or the other has gone extinct. Sometimes no -e variant is attested at all.

Most cases appear to involve /rr/ or a preceding labial sound, especially a stressed o, implying factors that either favoured the retention of an original /-o/ or (as more generally assumed by specialists) led to its coincidental redevelopment from /-e/.

Although this list intentionally excludes words recently borrowed from Spanish, such as agüelo or flonjo, there is nevertheless a possibility that at least some of the words reflect early contact with Aragonese.