Sri Aurobindo translates Prasha Upnaishad, Fifth Question, Verses 5-7: “But he who by all the three letters meditateth by this syllable, even by OM on the Most High Being, he in the Solar World of light and energy is secured in his attainings: as a snake casteth off its slough: so he casteth off sin, and the hymns of the Sama-veda escort him to the heaven of the Spirit. He from that Lower who is the density of existence beholdeth the Higher than the Highest of whom every form is one city. Whereof these are the verses: ‘Children of death are the letters when they are used as three, the embracing and the inseparable letters: but the wise man is not shaken: for there are three kinds of works, outward deed and inward action and another which is blended of the two, and all these he doeth rightly without fear and without trembling. To the earth the Rig-veda leadeth, to the skies the Yajur, but the Sama to That of which the sages know. Thither the wise man by resting on OM the syllable attaineth, even to that Supreme Quietude where age is not and fear is cast out by immortality.’ ”
These verses continue the examination of OM and its symbolism. The seeker who focuses on the integration of the symbolism of all three sounds A, U, M attains a complete realisation and the results in the material universe. The Upanishad indicates that these 3 are “children of death” because they are part of the transient, mutable creation that undergoes birth and death. The focus on one or another aspect of the reality, treated as separate from the others, is an incomplete realisation. This is the process of birth, growth and death that the soul participates in as it works to understand each aspect and then bring about the integration that is based in the reality of Oneness.
Beyond the understanding of the symbolism and significance of OM, there is also the occult impact of the vibration of the proper recitation of OM. The energetic influence of this recitation can calm the “mind stuff” (chitta), so that the seeker can reflect the higher truth without the distortion of a jumble of thoughts, emotions, perceptions, nervous impulses and desires that stir up the mind-stuff generally.
Sri Aurobindo, The Upanishads, Prashna Upanishad, pp.297-315