Although He (oṁkāra) has no material form, He is unlimitedly expanded, and He 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 Māṇḍūkya Upaniṣad. One should not foolishly interpret an Upaniṣadic description and say that because the Supreme Personality of Godhead "cannot" appear Himself in this material world in His own form, He sends His sound representation(oṁkāra) instead. Due to such a false interpretation, oṁkāra comes to be considered something material, and consequently oṁkāra is misunderstood and praised as being simply an exhibition or symbol of the Lord. Actually oṁkāra is as good as any other incarnation of the Supreme Lord.
Absolute knowledge (other books)
Other Books by Srila Prabhupada
Teachings of Lord Caitanya
By understanding oṁkāra one can become free from the duality of the material world and attain absolute knowledge.
Teachings of Lord Caitanya, Chapter 20: