Sri Aurobindo on OM

Volume 12 - The Word has its seed-sounds - suggesting the eternal syllable of the Veda, AUM.

A note on the Chhandogya Upanishad

"OM is the syllable (the Imperishable); one should follow after it as the upward song (movement); for with OM one sings (goes) upwards;

"The Chhandogya,... is to be a work in the right and perfect way of devoting oneself to the Brahman; its subject is the Brahman, but the Brahman as symbolised in the OM, the sacred syllable of the Veda, not therefore, the pure state of existence only, but that existence in all its parts... OM is the symbol and the thing symbolised.

Volume 13 - "the basic syllable OM, which is the foundation of all the perfect creative sounds of the revealed word; OM is the one universal formulation of the energy of sound and speech, that which contains and sums up, synthesises and releases, all the spiritual power and all the potentiality of Vak (speech, the goddess Speech) and Shabda (sound, vibration, word)

Volume 13 - Page 315 - The mantra of the divine consciousness brings its light of revelation, the Mantra of the divine Power, its will of effectuation, the Mantra of the divine Ananda is equal fulfilment of the spiritual delight of existence. All word and thought are an outflowing of he great OM, - OM, the Word, the Eternal Manifest in the forms of sensible objects; manifest in that conscious play of creative self-conception of which forms and objects are the figures, manifest behind in the self-gathered superconscient power of the Infinite, OM is the sovereign source, seed, womb of thing and idea, form and name - it is itself, integrally, the supreme Intangible, the original Unity, the timeless Mystery self- existent above all manifestation in supernal being."

AUM - A the spirit of the gross and external, Virat, U the spirit of the subtle and internal, Taijasa, M the spirit of the secret superconscient omnipotence, Prajna, OM the Absolute - Mandukya Upanishad.

Page 475 - OM is the symbol of the triple Brahman, the outward-looking, the inward of subtle and the superconscient causal Purusha (Conscious Being, Person)

Volume 23 - Page 745 - OM is the mantra, the expressive sound-symbol of the Brahman Consciousness in its four domains from the Turiya (the superconscient, the Absolute) to the external or material plane. The function of a mantra is to create vibrations in the inner consciousness that will prepare it for the realisation of what the mantra symbolises and is supposed indeed to carry within itself. The mantra OM should therefore lead towards the opening of the consciousness to the sight and feeling of the One Consciousness in all material things, in the inner being and in the supraphysical world, in the causal plane above now superconscient to us and, finally, the supreme liberated transcendence above all cosmic existence.

Volume 24 - The Triple Transformation - OM (golden) rising to the sky means the cosmic consciousness Supramentalised and rising towards the transcendent Consciousness.