Biblical title of honor, literally "father," used as an invocation of God, from Latin abba, from Greek abba, from Aramaic (Semitic) abba "the father, my father," emphatic state of abh "father." Also a title in the Syriac and Coptic churches.
It is used in the New Testament three times (Mark xiv. 36, Rom. viii. 15, Gal. iv. 6), in each instance accompanied by its translation, "Abba, Father," as an invocation of the Deity, expressing close filial relation. Either through its liturgical use in the Judeo-Christian church or through its employment by the Syriac monks, it has passed into general ecclesiastical language in the modified form of abbat or abbot .... [Century Dictionary]
late 14c., coniuracioun, "conspiracy, a plot, act of plotting" (senses now obsolete), also "a calling upon something supernatural, act of invoking by a sacred name, invocation of spirits, magic spell or charm," from Old French conjuracion "spell, incantation, formula used in exorcism" and directly from Latin coniurationem (nominative coniuratio) "a swearing together, conspiracy," in Medieval Latin "enchantment," noun of action from past-participle stem of coniurare "to swear together; conspire," from assimilated form of com "with, together" (see con-) + iurare "to swear," from ius (genitive iuris) "law, an oath" (see jurist).
interjection expressing various emotions (fear, surprise, pain, invocation, gladness, admiration, etc.), 1530s, from Middle English o, from Old French ô, oh or directly from Latin o, oh; a common Indo-European interjection (compare Greek ō; Old Church Slavonic and Lithuanian o; Irish och, Old Irish a; Sanskrit a). But it is not found in Old English (which had ea and translated Latin oh with la or eala) or the older Germanic languages except those that probably borrowed it from Greek or Latin.
The present tendency is to restrict oh to places where it has a certain independence, & prefer o where it is proclitic or leans forward upon what follows .... [Fowler]
Often extended for emphasis, as in Oh, baby, a stock saying from c. 1918; oh, boy (by 1917); oh, yeah (1924). Reduplicated form oh-oh as an expression of alarm or dismay is attested from 1944 (as uh-oh by 1935). Oh-so "so very" (often sarcastic or ironic) is by 1916. Oh yeah? "really? Is that so?" is attested from 1930.
magical formula used in conjuring, 1630s, earlier Hocas Pocas, common name of a magician or juggler (1620s); a sham-Latin invocation used by jugglers, perhaps based on a perversion of the sacramental blessing from the Mass, Hoc est corpus meum "This is my body." The first to make this speculation on its origin apparently was English prelate John Tillotson (1630-1694).
I will speak of one man ... that went about in King James his time ... who called himself, the Kings Majesties most excellent Hocus Pocus, and so was called, because that at the playing of every Trick, he used to say, Hocus pocus, tontus tabantus, vade celeriter jubeo, a dark composure of words, to blinde the eyes of the beholders, to make his Trick pass the more currantly without discovery. [Thomas Ady, "A Candle in the Dark," 1655]
Compare hiccus doccius or hiccus doctius, "formula used by jugglers in performing their feats" (1670s), also a common name for a juggler, which OED says is "conjectured to be a corruption of" Latin hicce es doctus "here is the learned man," "if not merely a nonsense formula simulating Latin." Also compare holus-bolus (adv.) "all at a gulp, all at once," which Century Dictionary calls "A varied redupl. of whole, in sham-Latin form." As a noun meaning "juggler's tricks," hocus-pocus is recorded from 1640s.