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.