Origins
The conjunction 'or' is the fundamental marker of alternatives in English, yet its history is more tangled than most function words. The modern monosyllable is not a simple inheritance from a single Old English ancestor but rather a convergence of two distinct words that merged during the Middle English period.
Old English had two primary ways to express alternatives. The conjunction 'oþþe' (or) was the standard disjunctive, used to link alternatives in ordinary speech: 'gōd oþþe yfel' (good or evil). Separately, the pronoun 'āhwæþer' (either, each of two, whether) — from 'ā' (ever) + 'hwæþer' (which of two) — could introduce paired alternatives: 'āhwæþer...oþþe' (either...or). During the late Old English and early Middle English periods, 'āhwæþer' was progressively shortened: first to 'āwþer,' then to 'ōþer,' and finally to 'or.' Simultaneously, 'oþþe' was weakening and falling out of use. The reduced form of 'āhwæþer' merged with or replaced 'oþþe,' and by the 13th century, 'or' had become the sole disjunctive conjunction.
The Old English 'oþþe' descends from Proto-Germanic *efþau (or), which is also the ancestor of German 'oder' (or), Dutch 'of' (or), and Old Norse 'eða' (or). The precise PIE etymology of *efþau is debated, but it may be related to PIE *éti (beyond, and) or to *h₁eti (furthermore). German 'oder' shows additional development with a dental suffix.
Proto-Indo-European Roots
The word 'either' preserves the fuller form of the Old English pronoun that contributed to 'or.' 'Either' is from Old English 'ǣghwæþer' (each of two, either), a compound of 'ǣg-' (each, ever) + 'hwæþer' (which of two). 'Whether' is from Old English 'hwæþer' (which of two), from PIE *kʷo-tero- (which of two), the same interrogative root that produced Latin 'uter' (which of two) and Sanskrit 'katará' (which of two). The concepts of 'or,' 'either,' and 'whether' are thus etymologically intertwined — all rooted in the ancient human need to choose between two possibilities.
The simplicity of 'or' in its modern form belies this complex history. Where Old English required the two-syllable 'oþþe' or the polysyllabic 'āhwæþer...oþþe' construction, Modern English achieves the same logical operation with a single open syllable. This extreme phonological reduction is characteristic of the highest-frequency grammatical words, which are spoken so often that they erode to their minimal possible form.