The Katha Upanishad has:
"The goal, which all Vedas declare, which all austerities aim at, and which humans desire when they live a life of conscience, I will tell you briefly it is aum"
"The one syllable [evākṣara, viz. aum] is indeed Brahman. This one syllable is the highest. Whosoever knows this one syllable obtains all that he desires.
"This is the best support; this is the highest support. Whosoever knows this support is adored in the world of Brahma." (1.2.15-17)[5]
The Chāndogya Upanishad (1.1.1-1) states:
om ity-etad akṣaram udgītham upāsīta / aum iti hy udgāyati / tasyopavyākhyānam
"The udgi:tā ["the chanting", that is, the syllable om] is the best of all essences, the highest, deserving the highest place, the eighth."
The Bhagavad Gi:tā (8.13) states that:
Uttering the monosyllable Aum, the eternal world of Brahman, One who departs leaving the body (at death), he attains the Supereme Goal (i.e., he reaches God).
In Bhagavad Gi:tā (9.17): Lord Krishna says to Arjuna - "I am the father of this universe, the mother, the support and the grandsire. I am the object of knowledge, the purifier and the syllable Oḿ. I am also the Ṛig, the Sāma and the Yajur Vedas."
The Bhagvad Gi:tā (17.23) has:
om tatsatiti nirdesho brahmanstrividhah samratah
"OM, tat and sat has been declared as the triple appellation of Brahman, who is Truth, Consciousness and Bliss."
In the following sūtra it emphasizes, "The repetition of Om should be made with an understanding of its meaning".[6]
In the book Om Chanting and Meditation Amit Ray states:
Om is not just a sound or vibration. It is not just a symbol. It is the entire cosmos, whatever we can see, touch, hear and feel. Moreover, it is all that is within our perception and all that is beyond our perception. It is the core of our very existence. If you think of Om only as a sound, a technique or a symbol of the Divine, you will miss it altogether. ..... Om is the mysterious cosmic energy that is the substratum of all the things and all the beings of the entire universe. It is an eternal song of the Divine. It is continuously resounding in silence on the background of everything that exists. [7]