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