Difference between "os, oris" and "os, ossis"

[Zacharias]

Is there a difference between the pronunciation of os (bone) and os (mouth) in Latin? Or any other differences between the two words or anything else you think I should know that's related?

 

[Matthaeus]

Yes, one has long o, ōs, meaning "mouth/face/countenance; the other os is short ŏs, meaning "bone". The plurals of these are ora, and ossa, respectively.

 

[Abbatiſſæ Scriptor]

The one meaning 'bone' appears with only one 's' in the nominative/accusative, but it actually has a double 'ss' stem, so its oblique cases were never rhoticised. Single 's' regularly becomes 'r' whenever it falls between two vowels, but double 'ss' survives as 'ss'. When we see a single 's' surviving between vowels in a Latin word, we can usually assume that it came into the language after the period of rhoticisation.

 

[Arca Defectionis]

Exactly what Scriptor said. The stem of "bone" is oss, that of "mouth" is os.

The nominative ending in the third declension is usually -s, so oss-s -> *osss -> os, since Latin words never end in double s.

oss-s -> os
oss-is
oss-i
oss-s -> os
oss-e

oss-a
oss-(i)-um (the ambiguity here arises from the fact that the nominative theoretically ends in -ss, satisfying the rules for i-stem nouns, though technically, since os is neuter, its being i-stem based on its nominative's ending in a double consonant is irregular anyways. The form ossum is more regular, derived from the actual nominative os.
oss-ibus
oss-a
oss-ibus

Os as in mouth has the stem os, which also receives and loses the -s: os-s -> *oss -> os.
The other forms have a single s in between two vowels, which is bad news in classical languages. In Latin, it means the switch to r; for example: os-is -> *osis -> oris

os-s -> os
os-is -> *osis -> oris
os-i -> *osi -> ori
os-s -> os
os-e -> *ose -> ore

os-a -> *osa -> ora
os-um -> *osum -> orum (note that here there is no chance of an i-stem ending)
os-ibus -> *osibus -> oribus
os-a -> *osa -> ora
os-ibus -> *osibus -> oribus

 

[Oups]

ŏs ossis : bone (short o)
ōs oris : mouth (long o)

 

[Abbatiſſæ Scriptor]

he nominative ending in the third declension is usually -s, so oss-s -> *osss -> os, since Latin words never end in double s.

I was wondering how similar this might be to the simplification of '-ts' to simple '-s' in the nominatives of ' t' stem nouns. In those, however, the the final 's' is a nominative case ending, which, like the accusative '-em', only attatches itself to m. or f. nouns in the athematic declensions. Athematic neuters take no ending whatever in the nom./acc. singular, so the analogy might not really hold here.