early 15c., "slanting, sloping, sideways; crooked, not straight or direct," originally of muscles or eyes, from Old French oblique (14c.) and directly from Latin obliquus "slanting, sidelong, indirect," which is perhaps from ob "against" (see ob-) + root of licinus "bent upward," from a PIE root meaning "to bend, be movable," the source of see limb (n.1). But De Vaan writes, "The etymology is unknown. Closest in form and meaning are līmus 'transverse' and sublīmis 'transverse from below upward', and the latter would be morphologically similar to oblīquus. Yet a root *lī- with different suffixes *-mo- and *-kwo- does not immediately make sense, and has no clear connections outside Italic." 

 Figurative sense of "indirect" is from early 15c. As a noun in anatomy in reference to a type of muscle the direction of whose fibers is oblique to the long axis of the body or to the long axis of the part acted, by 1838. Related: Obliquely; obliqueness.

early 15c., obliquite, "state of being slanted or twisted; crookedness (of eyes), also figurative, "moral transgression," from Old French obliquité (14c.), from Latin obliquitatem (nominative obliquitas) "slanting direction, obliquity," noun of quality from obliquus "slanting, sidelong, indirect" (see oblique).

"blot out, cause to disappear, remove all traces of, wipe out," c. 1600, from Latin obliteratus, past participle of obliterare "cause to disappear, blot out (a writing), erase, efface," figuratively "cause to be forgotten, blot out a remembrance," from ob "against" (see ob-) + littera (also litera) "letter, script" (see letter (n.)). The verb was abstracted from the phrase literas scribere "write across letters, strike out letters." Related: Obliterated; obliterating.

"act of obliterating or effacing, a blotting out or wearing out, fact of being obliterated, extinction," 1650s, from Late Latin obliterationem (nominative obliteratio), noun of action from past-participle stem of obliterare "cause to disappear, blot out (a writing)," figuratively "cause to be forgotten, blot out a remembrance" (see obliterate).

late 14c., oblivioun, "state or fact of forgetting, forgetfulness, loss of memory," from Old French oblivion (13c.) and directly from Latin oblivionem (nominative oblivio) "forgetfulness; a being forgotten," from oblivisci (past participle oblitus) "forget," which is of uncertain origin.

Perhaps originally "even out, smooth over, efface," from ob "over" (see ob-) + root of levis "smooth," also "rubbed smooth, ground down," from PIE *lei-w-, from root *(s)lei- "slime, slimy, sticky" (see slime (n.)); compare obliterate. But de Vaan and others find that "a semantic shift from 'to be smooth' to 'to forget' is not very convincing." However no better explanation has emerged.

Meaning "state or condition of being forgotten or lost to memory" is from early 15c. In English history, the Acts of Oblivion use the word in the sense of "intentional overlooking" (1610s), especially of political offenses. Related: Obliviously; obliviousness.

Oblivion is the state into which a thing passes when it is thoroughly and finally forgotten. ... Forgetfulness is a quality of a person: as a man remarkable for his forgetfulness. ... Obliviousness stands for a sort of negative act, a complete failure to remember: as a person's obliviousness of the proprieties of an occasion. [Century Dictionary]

mid-15c., "forgetful, disposed to forget, heedless," from Latin obliviosus "forgetful, that easily forgets; producing forgetfulness," from oblivio "forgetfulness, a being forgotten"(see oblivion). Meaning "unaware, unconscious (of something)" is by 1862; it formerly was regarded as erroneous, but this is now the main meaning and the word has lost its original sense of "no longer aware or mindful." Properly it should be used with to, not of. Related: Obliviously; obliviousness.

"elongated, having one principal axis considerably longer than the others," early 15c., from Latin oblongus "more long than broad," originally "somewhat long," from ob "in front of; towards" here perhaps intensive (see ob-) + longus "long" (see long (adj.)). As a noun, "an oblong figure," from c. 1600.

mid-15c., obloquie, "evil speaking, slander, calumny, derogatory remarks," from Medieval Latin obloquium "speaking against, contradiction," from Latin obloqui "to speak against, contradict," from ob "against" (see ob-) + loqui "to speak," from PIE root *tolkw- "to speak." Related: Obloquious.

"a keeping silent, a becoming willfully mute or obstinately speechless," 1640s, from Late Latin obmutescere "to become dumb or mute," from ob "against, before," here perhaps intensive (see ob-) + mutescere "to grow dumb," an inchoative verb formed from mutus "silent, speechless, dumb" (see mute (adj.)).