As far as the oṁkāra (praṇava) is concerned, it is considered to be the sound incarnation of the Supreme Personality of Godhead. As such, oṁkāra is eternal, unlimited, transcendental, supreme and indestructible. He (oṁkāra) is the beginning, middle and end, and He is beginningless as well. When one understands oṁkāra as such, he becomes immortal. One should thus know oṁkāra as a representation of the Supreme situated in everyone's heart. One who understands oṁkāra and Viṣṇu as being one and the same and all-pervading never laments in the material world, nor does he remain a śūdra.
Although He (oṁkāra) has no material form, He is unlimitedly expanded and has unlimited form. By understanding oṁkāra one can become free from the duality of the material world and attain absolute knowledge. Therefore oṁkāra is the most auspicious representation of the Supreme Lord. Such is the description given by the Māṇḍūkya Upaniṣad. One should not foolishly interpret an Upaniṣadic description and say that it is because the Supreme Personality of Godhead cannot appear Himself in this material world in His own form that He sends His sound representation (oṁkāra) instead. Due to such a false interpretation, oṁkāra has come to be considered something material, and consequently oṁkāra is misunderstood and eulogized as being simply an exhibition or symbol of the Lord. Actually oṁkāra is as good as any other incarnation of the Supreme Lord.
The Lord has innumerable incarnations, and oṁkāra is one of them, in the form of a transcendental syllable. As Kṛṣṇa states in the Bhagavad-gītā (9.17): "Among vibrations, I am the syllable om." This means that oṁkāra is nondifferent from Kṛṣṇa. Impersonalists, however, give more importance to oṁkāra than to the Personality of Godhead, Kṛṣṇa. But the fact is that any representational incarnation of the Supreme Lord is nondifferent from Him. Such an incarnation or representation is as good spiritually as the Supreme Lord. Oṁkāra is therefore the ultimate representation of all the Vedas. Indeed, the Vedic mantras or hymns have transcendental value because they are prefixed by the syllable om. The Vaiṣṇavas interpret oṁkāra, a combination of the letters a, u and m, as follows: By the letter a, Kṛṣṇa, the Supreme Personality of Godhead, is indicated; by the letter u, Kṛṣṇa's eternal consort, Śrīmatī Rādhārāṇī, is indicated; and by the letter m, the living entity, the eternal servitor of the Supreme Lord, is indicated. Śaṅkara has not given such importance to oṁkāra. But such importance is given in the Vedas, the Rāmāyaṇa, the Purāṇas and the Mahābhārata, from beginning to end. Thus the glories of the Supreme Lord, the Supreme Personality of Godhead, are declared.