“Oṃ — this whole world is that syllable. Here is a further explanation of it. All that is past, present, and future is simply Oṃ.”
— Māṇḍūkya Upaniṣad, 1
The sound
Oṃ is first a sound, and the sound is what the tradition actually teaches. Written it is three letters — a-u-m — and the teaching is that these are not three but one: a is the vowel that arises at the back of the throat (where sound begins), u is the rounding of the lips as the sound moves forward, m is the hum that closes the lips and reverberates in the skull. The three contain every articulation the human voice can make. The fourth part — amātra, the measureless — is the silence into which the sound dissolves.
This is what the Upanishads mean when they teach that Oṃ is the whole: waking (a), dreaming (u), dreamless sleep (m), and the Fourth (turīya) — Brahman itself, which all three arise from and return to.
In Hindu tradition
In Hinduism Oṃ is called praṇava — “the sound that precedes.” Every Vedic chant opens and closes with it. Every mantra is held to be a particularization of it. The Bhagavad Gītā names Oṃ directly: “I am the syllable Oṃ in all the Vedas” (9.17); “Utter the single syllable Oṃ, the Brahman, remembering me” (8.13).
Advaita Vedānta took Gauḍapāda’s commentary on the Māṇḍūkya as its foundational text. For Advaita, Oṃ is not a symbol pointing at Brahman; it is what Brahman sounds like when sound is possible. The meditative practice is to vocalize a-u-m slowly, listening to each phase, and to rest in the silence that follows as the turīya that was always there.
In Buddhism
Oṃ entered Buddhism through the Mahāyāna, where it opens many mantras. The most widely known is the six-syllable mantra of Avalokiteśvara — Oṃ maṇi padme hūṃ — which the Tibetan tradition holds to contain the entire teaching of the Mahāyāna. Oṃ here opens the practice; it is the recognition of the awakened ground that the practice will unfold from.
Buddhism does not, on the whole, give Oṃ the cosmological weight the Upanishads do. The Buddha’s teaching is not that there is a primordial sound called Brahman; but the efficacy of sacred sound — the physical fact that certain vocalizations done correctly settle the mind — is preserved and transformed.
In Jainism and Sikhism
Jainism gives Oṃ a five-fold reading: the five letters A-A-A-U-M stand for the five supreme beings (pañca-parameṣṭhi) — the awakened, the liberated, the teachers, the masters, the monks. The syllable is the condensation of the Namokāra Mantra.
In Sikhism, Guru Nanak’s opening syllable of the Japji Sahib — Ik Onkār (ੴ) — begins with a variant: Ik (one) + Oṃ + kār (doing, the one-in-action). It is the first character of the Guru Granth Sahib and the seal of the Sikh tradition. The meaning Nanak gives it: the One Reality that is formless, timeless, without fear, without enmity, self-existent, known through the grace of the Guru.
The glyph
The written symbol ॐ is a Devanagari ligature. Its top curve is often read as a, its lower curve as u, its tail as m; the dot (bindu) above is the silent fourth, turīya; the crescent below the dot is the nāda, the flowing sound. Every element of the syllable is in the glyph. Iconographically it is one of the most recognized religious symbols in the world — which is also why, in the present, it appears on yoga-studio signs and tattoo arms with no connection to the teaching it carries. The atlas notes this without scolding.
As practice
Japa — the soft vocal or silent repetition of Oṃ — is one of Hindu practice’s most ancient forms. It is done counted on a mālā of 108 beads, or silently in time with the breath, or as audible chant in groups. Practitioners report that extended chanting of Oṃ settles the nervous system in ways that can be measured (slowed heart rate, reduced default-mode-network activity in fMRI studies — see Kalyani et al., 2011, in International Journal of Yoga). The tradition knew this without instruments.
Patañjali’s Yoga Sūtra I.27 states it plainly: tasya vācakaḥ praṇavaḥ — “its expression is Oṃ” — where “its” refers to Īśvara, the principle that gathers and directs consciousness. Repetition of Oṃ with contemplation of its meaning is, for Patañjali, the royal method.
Across the world
The syllable’s cognates and close parallels are many and should not be collapsed into it: the Hebrew Amen, the Christian Amen, the Islamic Āmīn — all from a Semitic root meaning “truly, so be it” — are related by function (the solemn seal of a prayer) and sound-shape but not by etymology. The Sufi Hū — the dhikr breath-syllable that names the divine pronoun — is another sacred open-throated sound; again, a parallel, not a derivation.
The atlas records parallels as parallels.
“In the beginning was the Word.”
— John 1:1
“The word that begins all words is Oṃ.”
— Māṇḍūkya Upaniṣad, tradition of recitation