'Or' collapsed from two Old English words — 'oththe' and 'ahwaether' — merging into one tiny syllable of choice.
Used to link alternatives; used to introduce a synonym or explanation of a preceding word or phrase.
From Middle English 'or,' a contracted form arising from the collision of two Old English disjunctive words: 'oththe' (or) from Proto-Germanic *efthau (or, either) and the reduced form of 'ahwaether' (either, whichever of two), which shortened through 'other' to 'or.' The Proto-Germanic *efthau connects to Gothic 'aiththau' (or), Old Saxon 'efdo,' and Old High German 'eddo.' Ultimately the Proto-Indo-European disjunctive particle *h2e (or, but) underlies several branches
Modern English 'or' is actually a merger of two different Old English words: 'oþþe' (or) and a shortened form of 'āhwæþer' (either of two). The two words collapsed into a single syllable during Middle English. Meanwhile, 'either' and 'whether' are also related — all from the PIE root *kʷo- (who, which), expressing the concept of choice between alternatives.