late 14c., "state or fact of forgetting," from Old French oblivion (13c.) and directly from Latin oblivionem (nominative oblivio) "forgetfulness; a being forgotten," from oblivisci (past participle oblitus) "forget," 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 of being forgotten or lost to memory" is early 15c. 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]