A mantra of Yajur Vedam salutes the Sacred syllabale OM and comments on the significance of Pranavam this way: " O, Men, by me, the resplendent protector, the matter , the face of the eternal cause is protected. The spirit yonder is the Sun, the spirit dwelling inside the Sun is ME. I am vast like the atmosphere,greatest of all in merit,action and nature.OM IS MY NAME."
A Mantra of the Maha Narayanopanishad echoes the above message of Yajur Vedam as Follows: " Aditya, the supreme cause of this Universe isa denoted by the syllable Om.That form of Sun is Brahman. That is the pervading cause of All. That is water,fire, flavor and ambrosia (Immortality) . The three Vyahritis (Bhu:, Bhuva:, and Suvaha:) representing the three worlds and the PRANAVA representing the cause of the universe denote Brahman." Maha Narayanopanishad states elsewhere that the One syllable Om is Brahman. It also reveals that the Pranavam is recited and reflected upon to acheive the union with Brahman, who exists as the manifold universe.
The Narayanopanishad also contains a reference to "AUM" in two places.
"Omithyagrae Vyahararaeth" and "OMithyaekAksharam" . The latter is supposed to be the only mantram for Sri Vaishnavas.
Eka Mantram: OMithyaekAksharam
Eka NAmam: Devaki PutrOnamaha
Eka Seva: Devaki PutrOseva
The all-encompassing nature of Pranavam is explained by the Mandukya Upanishad mantra as follows: " OM(AUM) the syllable is the whole world. It is the past , present and the future. Everything is just the word AUM. " Pranavam is recognized here as the resonating sound embodiment of the supreme being ,who pervades Space, Time and Form Completely.
Prsana Upanishad points out that AUM is recommended for the meditation on and the worship of Supreme Reality in its Saguna Brahman Form with attributes and the Nirguna Brahman Form without attributes . In a Katopanishad Mantra, The Lord of Death (Yama Raja) reveals to the aspirant Nachiketas that " this word (Om) is verily Brahman, this word is verily the highest . He ,who knows this word obtains ,verily, whatever he desires."
A passage in the Brugu Valli of the Taittiriya Upanishad explains to us the benefits accrued from recognizing and worshipping Om as Brahman. It says: " Let him, the aspirant, meditate on the supreme (Aum) as the support; he becomes well supported. Let him worship Aum (Brahman) as great; he becomes great. Let him worship it as Mind; he becomes thoughtful. Let him worship Aum as Nama(obesiance); to him all desires shall come to pay homage. Let him meditate upon that as the Supreme; he comes to supermacy in Life."
At the conclusion of the Vaiswa Deva Mantra passage of the Narayana Valli of the same Upanishad, we get another insight into the significance of Om. " Brahman is the meaning of Omkara. It is the embodiment of All that is subtle,gross and casual. My salutations to that Brahman! That Omkaram resides in the caves of the hearts of the human beings. O Brahman(Pranavam)! You are the sacrificial ritual, You are the Vashatkaram (Power). You are Indra, Rudra, Vishnu, Brahma, Water, Shining Stars and Ambrosia (Immortality).It is for these reasons that the Brahadaranyaka Upanishad points out again that the Pranavam is identical to Brahman and should therefore be worshipped as Brahaman.In the fourth Anuvakha of Siksha Valli of the Taittiriya Upanishad, Om is pointed out as the most sacred Mantra for the aspirant to illuminate all his faculties including his own intellect(Medha Sakti) . Pranavam is recognized here as the sheath of Brahman and the invocation of OM is declared as the equivalent of the invocation of Brahman.
The Syllable AUM arises from the Sanskrit root "AV" , which means to protect.. AUM is understood therefore to protect the Universe and all of its inhabitants. This role of Pranavam makes all living beings bow before the Supreme Lord: " Pranan sarvan Paramatmani Pranamayati iti Pranavam" is thus the other defenition of Pranavam. AUM Or Om or Pranavam is also known as the Brahma Vidya Mantram. In this context, it is recognized as the Mantra that leads the Sadhakas (aspirants) to the other shore (Tara) of the ocean of Samsara(viz) , the abode of the Supreme Lord.
Aum is known as the Vedic Pranavam to distinguish it from the other Pancha Pranavas known as EYM(Saraswati worship), HREEM ( Sri Vidya Worship) ,SREEM ( Maha Lakshmi Worship), Kreem (Kali Worship), and KLEEM (Kama Kala Worship). The above Pancha Pranavams are known as Tantric Pranavam; It is generally agreed that the Tantras are not totally disconnected to the Vedas , since Vedic Sookthams such as Sri Sooktham, Durga Sooktham , are attached to the powers behind these Pancha Pranavams. These are meditated upon to get specific boons such as Knowledge from Saraswati, Worldly wealth from Maha Lakshmi etc. AUM on the other hand is recited and reflected upon to get the Supreme Brahma Gyanam.
The syllable AUM has a four fold division according to Vedic Niruktham or Etymology. Nrsimha Uttara Tapini Upanishad explains how this four fold Vyashti (Constituents) from the Samashti (Whole). A, U, M and the Ardha Matra (Half Letter) at the end of the sacred syllable represent the four constituents of Pranavam. The letter "A" represents the Rg Veda. It represents the terrrestrial world surrounded by the seven seas according to this Upanishad. The Garhapatya Agni (Householder"s fire ) is its Agni. The letter"U" represents the Yajur Veda and the intermediary world. The second Vedic fire , Dakshinagni is its Agni. The letter "M" represents the celestial world and the Sama Veda. Its fire is the Ahavagni. The half letter or the nasalization at the end of the three letters, "A, U, M " represents the Lunar World and the Atharva Veda. The Ekarshi fire of the Atharva Ritual is its Agni. In its Samashti form, The Pranavam thus contains all the 4 Vedas and their immortal hymns from which the Brahman , the truth symbolized by the sound "Om" arises.
The letters A,U, M are accepted here as representing Vishnu, Lakshmi and the Jeevan or the individual soul. The Jeevan represented by M is recognized as the Jnata (the Knower), Karta (the Doer), Bhokta (the Enjoyer) and the Sesha (the subservient being of the Lord Narayana and His Consort) . The Fourth part, the nasalization is recognized as the eternal, subservient relationship of the Jeevan to the Divine Couple, who represent Eka Seshitvam.
The Narada Parivrajaka Upanishad goes one step further and explains the 8 fold structure of Pranavam. The 16 fold and the 128 fold divisions of Pranavam are also explained by Brahma to His son , Sage Narada in the same upanishad. The 16 fold division adds to the above four subdivisions following additional 12 subdivisions: Bindu,Nada, Kaala(Time), Kaaladita( Beyond Time), Santi (Tranquility), Santyatita(Beyond tranquility) , Unmani (the state of absence of Gross Mind),Mnaonmani(State of Absence of Subtle Mind), Puri,Madhyama, Pasyanti and Para . The last four are associated with the formation of Speech. Lord Brahma explains to His son that this Pranavam is Turya-Turya or the Supreme Bliss and identifies it as the Prop of ALL. He points out that OM should be sought by every one and that It is the beginning of every thing and the bestower of the final Beautitude.
It says: " Making the Self , the lower kindling stick and the Pranavam , the upper kindling stick and by the practise of vigorous rubbing (Meditation), the sage may realize the hidden effulgent being as the Fire hidden in the sticks. As Oil present in the Sesamum seeds, ghee in curds, water in the underground springs, and Fire in the kindling sticks, so is the Supreme Being realized in the individual self by one, who visualizes Him thru Truth and Penance."