Chandogya Upanishad, one of the oldest Upanishad has been written during the Vedic Brahmana period. Chandogya Upanishad is associated with Sama Veda. Chandogya Upanishad is the part of Chandogya Brahaman that contains ten chapters. The second chapter of Chandogya Upanishad contains twenty-four khandas. The First Khanda of Chapter One of Chandogya Upanishad deals with the meditation on the Om Mantra. According to it the syllable Om, called the Udgitha, should be meditated upon. People, who sing the Udgitha, begin with Om. It states that the essence of all these beings is the earth; the essence of the earth is water; the essence of water is plants; the essence of plants is a person; essence of a person is speech; the essence of speech is the Rig Veda; essence of the Rig Veda is the Sama Veda; the essence of the Sama Veda is the Udgitha which is Om. The Udgitha i.e. the Om is the best of all essences, the supreme and deserves the highest place. The Speech is the Rik; the vital breath (prana) is the Saman; the syllable Om is the Udgitha. Speech and the prana, or the Rik and the Saman, form a couple. That couple becomes united in the syllable Om. When a couple come together they fulfil each other`s desire. One who is aware of this and meditates on the syllable Om, the Udgitha, becomes a fulfiller of desires. He gets rid of all sorrows and experiences immense joy. This syllable Om is used to give assent, for wherever one assents to something; one says Om. He who knows this and meditates on the syllable Om, the Udgitha, becomes, indeed, a gratifier of desires.
By means of this syllable the threefold knowledge can be gained. When adhvaryu priest gives an order in a sacrifice, he says Om. When the hotri priest recites the hymn, he says Om. When the udgatri priest sings the Saman, he also recites the Om Mantra. This is done for the glory of the Imperishable Atma by the greatness of that syllable. One who is aware of this reaps the fruits of his worship and sacrifice.
It may be contended that he who knows this true meaning of the syllable Om and he who does not, perform the same sacrifice and therefore must reap the same fruit. But it has been mentioned that the work that is done with knowledge, faith produces more powerful fruit.
