ore rotundo adv.

1620s, "uttered by the mouth or in words;" 1650s, "of or pertaining to the mouth," from Late Latin oralis, from Latin os (genitive oris) "mouth, opening, face, entrance," from PIE *os- "mouth" (source also of Sanskrit asan "mouth," asyam "mouth, opening," Avestan ah-, Hittite aish, Middle Irish a "mouth," Old Norse oss "mouth of a river," Old English or "beginning, origin, front").

Os was the usual word for "mouth" in Latin, but as the vowel distinction was lost it became similar in sound to os "bone" (see osseous). Thus bucca, originally "cheek" but used colloquially as "mouth," became the usual word for "mouth" (see bouche).

The psychological meaning "of the mouth as the focus of infantile sexual energy" (as in oral fixation) is attested from 1910. The sex-act sense is first recorded 1948, in Kinsey. As a noun, "oral examination," attested from 1876. Related: Orally (c. 1600); orality. 

"round, spherical, globular; rounded out, bulbous," 1705, from Latin rotundus "rolling, round, circular, spherical, like a wheel," from rota "wheel" (see rotary). Earlier was rotound (1610s); rotounde (early 15c.); rotundal (1620s), rotundious (1620s). In reference to a full-toned style of oratory by 1830, after Horace's ore rotundo in "Poetics," from the roundness of the mouth in pronouncing.

in elocution, "characterized by strength, fullness, richness, and clearness," 1792, from Latin ore rotundo "in well-rounded phrases," literally "with round mouth" (see ore rotundo).

The odd thing about the word is that its only currency, at least in its non-technical sense, is among those who should most abhor it, the people of sufficient education to realize its bad formation; it is at once a monstrosity in its form & a pedantry in its use. [Fowler]