oral (adj.)

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. 

Definitions of oral
oral (adj.)
using speech rather than writing;
an oral agreement
an oral tradition
Synonyms: unwritten
oral (adj.)
of or relating to or affecting or for use in the mouth;
an oral thermometer
an oral vaccine
oral hygiene
oral (adj.)
of or involving the mouth or mouth region or the surface on which the mouth is located;
the oral mucous membrane
the oral cavity
the oral surface of a starfish
Synonyms: buccal
oral (adj.)
a stage in psychosexual development when the child's interest is concentrated in the mouth; fixation at this stage is said to result in dependence, selfishness, and aggression;
oral (n.)
an examination conducted by spoken communication;
Synonyms: oral exam / oral examination / viva voce / viva