"Western film" (featuring horse-riding cowboys and Indians), 1946, American English, from oat, as the typical food of horses. Oats opera (on the model of soap opera) is by 1937 in U.S. slang.

Middle English oth, from Old English að "judicial swearing, solemn appeal (to deity, sacred relics, etc.), in witness of truth or a promise," from Proto-Germanic *aithaz (source also of Old Norse eiðr, Swedish ed, Old Saxon, Old Frisian eth, Middle Dutch eet, Dutch eed, German eid, Gothic aiþs "oath"), from PIE *oi-to- "an oath" (source also of Old Irish oeth "oath"). Common to Celtic and Germanic, possibly a loan-word from one to the other, but the history is obscure and it may ultimately be non-Indo-European. In reference to careless invocations of divinity, from late Old English.

"the coarsely ground meal of oats," late 14c., ote-mele, from oat + Middle English mele (see meal (n.2)).

"a walking about," c. 1600, from Latin obambulationem (nominative obambulatio) "a going or walking about," noun of action from past-participle stem of obambulare "to go or walk about, walk past, walk near," from ob "about" (see ob-) + ambulare "to walk, go about" (see amble (v.)).

musical instruction, used of accompaniments (especially by a single instrument, to a vocal piece), "so important that it cannot be omitted," 1724, from Italian obbligato, literally "obligated," from Latin obligatus, past participle of obligare "to bind" (see oblige). As a noun, "an accompaniment of independent importance," by 1817.

"stubbornness," especially "state of being hardened against moral influences, rebellious persistence in wickedness," 1590s, from obdurate + abstract noun suffix -cy.

mid-15c., "stubborn, inexorable, unyielding; hardened," especially against moral influences; "stubbornly wicked," from Latin obduratus "hardened," past participle of obdurare "harden, render hard; be hard or hardened; hold out, persist, endure," in Church Latin "to harden the heart against God," from ob "against" (see ob-) + durare "harden, render hard," from durus "hard," from PIE *dru-ro-, suffixed variant form of root *deru- "be firm, solid, steadfast." Variant opturate is from early 15c. in medicine in a literal sense of "stopped, obstructed."  Related: Obdurately; obdurateness.

c. 1400, obduracioun, "hard-heartedness; defiant impenitence," from Late Latin obdurationem (nominative obduratio) "a hardening," noun of state from past-participle stem of Latin obdurare "harden, render hard; be hard or hardened" (see obdurate).

"sorcery, witchcraft" among Africans in Africa and the West Indies, 1760, from a West African word, such as Efik (southern Nigeria) ubio "a thing or mixture left as a charm to cause sickness or death," Twi ebayifo "witch, wizard, sorcerer."