Oath

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Old English āth, of Germanic origin; related to Dutch eed and German Eid .

wiktionary

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From Middle English ooth, oth, ath, from Old English āþ(“oath”), from Proto-Germanic *aiþaz(“oath”), from Proto-Indo-European *h₁óytos(“oath”). Cognate with Scots aith, athe(“oath”), North Frisian ith, iss(“oath”), West Frisian eed(“oath”), Dutch eed(“oath”), German Eid(“oath”), Swedish ed(“oath”), Icelandic eið(“oath”), Latin ūtor(“use, employ, avail”), Old Irish óeth(“oath”).

etymonline

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oath (n.)

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.