AUM is the Great God Who is omnipresent (like ether)." YAJUR VEDA 40: 17.
"He alone, whose name is AUM, Who is Immortal, is worthy of our adoration and none other." CHHANDOGYA UPANISHAD
"All the Vedas and the Shastraas declare AUM as the primary and natural name of God. All others are His secondary names." MANDUK UPANISHAD 1:1
"He Whom all the Vedas declare worthy of homage, Whom all devotion and righteous actions lead to, and for Whose realization, the life of Brahmacharya(chastity) is led, is called AUM. KATH UPANISHAD 7:15.
"He , Who is the Teacher of all, subtler than the subtler tan the subtle, Resplendent, Who can be known through understanding begotten of Samadhi, i.e., 'superior condition' of the soul, when the mind is perfectly concentrated by means of psychical practices, is the Great Being." "Being All-glorious, some call Him Agni. Being the Embodiment of all true knowledge, other call Him Manu. Other call Him Indra, being All-powerful and Protector of all. Others Pranaa, as the source of all-life. Others, again, call Him Brahma,* the Greatest of all beings." MANU 12: 122, 123.