Om

The oldest sustained analysis of the syllable is also the shortest text in the Upanishadic corpus, and it does something unexpected with it.

The Mandukya Upanishad runs to twelve verses. It takes the syllable apart into three sounds, a, u, m, and matches each to a state of consciousness: a to waking, in which awareness is turned outward; u to dream, turned inward; m to deep dreamless sleep, in which there is no division into knower and known. It then names a fourth, turiya, which is not a fourth sound but the silence the three resolve into, and identifies that with the self.

The scheme is not decorative. It is an argument that the structure of a sound models the structure of consciousness, and it is the basis of the commentarial tradition that follows, particularly Gaudapada's verses on the text and Shankara's commentary on those.

A quite different account comes from the Chandogya Upanishad, which opens by identifying the syllable with the udgitha, the sung portion of the Vedic sacrifice, and builds its case from ritual rather than from psychology. That older ritual layer is where the syllable actually comes from: it functions in the Brahmanas as a formula of assent and response in liturgy, comparable in position to amen, before it becomes an object of philosophical attention.

The spelling in English books varies. Older works usually write Aum, reflecting the analysis into three; most modern works write Om, reflecting the sound. Neither is more correct; the Sanskrit is a single character.

It travelled further than any other element of Vedic religion, and each tradition that took it up supplied its own account of what it holds.

Buddhism uses it at the head of a great many mantras, most famously the six-syllable formula of Avalokiteshvara, and Vajrayana pairs it with two further syllables as a triad addressed to body, speech and mind. Buddhist commentary does not accept the Upanishadic identification with an unchanging self, since the doctrine of not-self rules that out, and reads the syllable instead in terms of the enlightened body or of the whole of sound.

Jainism reads it as an acronym. The five syllables of the great homage formula, arihant, ashiri or siddha, acharya, upadhyaya, muni, are taken to compress into the one sound, so that uttering it salutes all five categories of the venerable at once. This is a specifically Jain derivation and it is not a variant of the Hindu one.

Sikhism opens its scripture with Ik Onkar, one Onkar, and the numeral is doing the work: the point being made is oneness, and the syllable is inflected into a statement about a single formless creator rather than left as a bare sound.

What none of this is, is a shared meaning. Four traditions use the same sound and give four incompatible accounts of it, which is a good illustration of how a formula can be transmitted intact while what it is understood to say changes completely. The reduction of it to a generic sound of the universe in twentieth-century Western usage flattens all four.