Sri Aurobindo translates Chhandogya Upanishad, Chapter One, Section One, Verses 1-3: “Worship ye OM, the eternal syllable, OM is Udgitha, the chant of Sama-veda; for with OM they begin the chant of Sama. And this is the exposition of OM. Earth is the substantial essence of all these creatures and the waters are the essence of earth; herbs of the field are the essence of the waters; man is the essence of the herbs. Speech is the essence of man, Rig-veda the essence of Speech, Sama the essence of Rik. Of Sama OM is the essence. This is the eighth essence of the essences and the really essential, the highest and it belongeth to the upper hemisphere of things.”
Sri Aurobindo notes: “The Chhandogya, we see from its first and introductory sentence, is to be a work on the right and perfect way of devoting oneself to the Brahman; the spirit, the methods, the formulae are to be given to us. 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 the Universal Existence only, but that Existence in all its parts, the waking world and the dream self and the sleeping, the manifest, half-manifest and hidden, Bhurloka, Bhuvar and Swar, –the right means to win all of them, enjoy all of them, transcend all of them, is the subject of the Chhandogya. OM is the symbol and the thing symbolised. It is the symbol, aksaram; the syllable in which all sound of speech is brought back to its wide, pure indeterminate state; it is the symbolised, aksaram, the changeless, undiminishing, unincreasing, unappearing, undying Reality which shows itself to experience in all this change, increase, diminution, appearance, departure which in a particular sum and harmony of them we call the world, just as OM, the pure eternal sound-basis of speech shows itself tot he ear in the variations and combinations of impure sound which in a particular sum and harmony of them we call the Veda.”
In the Vedic view of things, there is an upper hemisphere of pure Existence-Consciousness-Bliss that is the true source, continent and basis of all that manifests. The image of the tree with its roots above and its branches and leaves below characterises this upper hemisphere as the most essential, the source of the world and all its forms. OM is the sound-body of this highest essential truth. By worshiping OM the worshiper becomes one with this highest truth of existence, which transcends all, contains all and embodies all that exists.
Sri Aurobindo, The Upanishads, Chhandogya Upanishad, pp.349-366