oryoudaoicibaDictYouDict[or 词源字典]
or: [12] The Old English word for ‘or’ was oththe. This appears to have been altered in the early Middle English period to other, probably due to the influence of similar words denoting ‘choice between alternatives’ and ending in -er (notably either and whether). Other was soon contracted to or, but it did not finally die out until the 15th century.
[or etymology, or origin, 英语词源]
or (conj.)youdaoicibaDictYouDict
c. 1200, from Old English conjunction oþþe "either, or," related to Old Frisian ieftha, Middle Dutch ofte, Old Norse eða, Old High German odar, German oder, Gothic aiþþau "or." This was extended in early Middle English (and Old High German) with an -r ending, perhaps by analogy with "choice between alternative" words that ended thus (such as either, whether), then reduced to oþþr, at first in unstressed situations (commonly thus in Northern and Midlands English by 1300), and finally reduced to or, though other survived in this sense until 16c.

The contraction took place in the second term of an alternative, such as either ... or, a common construction in Old English, where both words originally were oþþe (see nor).