os

Os, oris is third declension neuter. Nominative and accusative are os, the plural is ora, and the stem is or-. It means the mouth, and then by extension the face, the opening of anything, and the voice: uno ore, "with one voice", is unanimity.

Here is the trap, and it is a real one. There is a SECOND os in Latin: os, ossis, also neuter, meaning "bone", with plural ossa and genitive plural ossium. In the nominative and accusative singular the two are spelled identically, and the only thing that tells them apart is the genitive, oris against ossis. In speech the vowel of the mouth-word is long and that of the bone-word short, which our unmacroned spelling does not show. This is the clearest argument on the whole site for learning the genitive with every noun: without it, os is simply ambiguous.

From the mouth-word English took oral, orifice, oracle, orator and adore. From the bone-word it took osseous, ossify and ossuary.