The material world is a sum total manifestation of the three modes of material nature, technically called the pradhāna. The Vedic hymns sarvaṁ hy etad brahma (Māṇḍūkya Upaniṣad 2), tasmād etad brahma nāma-rūpam annaṁ ca jāyate (Muṇḍaka Upaniṣad 1.2.10), and, in the Bhagavad-gītā (14.3), mama yonir mahad brahma indicate that everything in the material world is a manifestation of Brahman; and although the effects are differently manifested, they are nondifferent from the cause. In the Īśopaniṣad it is said that everything is related to the Supreme Brahman, or Kṛṣṇa, and thus everything belongs to Him only.
(Bhagavad-Gita-----5:10-----purport).

The example of misunderstanding a rope to be a snake is mentioned in the Māṇḍūkya Upaniṣad, but it is meant to explain the error of identifying the body with the soul. Since the soul is actually a spiritual particle, as confirmed in the Bhagavad-gītā (mamaivāṁśo jīva-loke), it is due to illusion (vivarta-vāda) that a human being, like an animal, identifies the body with the self. This is a proper example of vivarta, or illusion. The verse atattvato ’nyathā-buddhir vivarta ity udāhṛtaḥ describes such an illusion.
(Sri Caitanya Caritamrta-----1:7:122-----purport).

All potencies are invested in the holy vibration of the holy name of the Lord. There is no doubt that the holy name of the Lord, or oṁkāra, is the Supreme Personality of Godhead Himself. In other words, anyone who chants oṁkāra and the holy name of the Lord, Hare Kṛṣṇa, immediately meets the Supreme Lord directly in His sound form. In the Nārada-pañcarātra it is clearly said that the Supreme Personality of Godhead Nārāyaṇa personally appears before the chanter who engages in chanting the aṣṭākṣara, or eight-syllable mantra, oṁ namo nārāyaṇāya. A similar statement in the Māṇḍūkya Upaniṣad declares that whatever one sees in the spiritual world is all an expansion of the spiritual potency of oṁkāra.
(Sri Caitanya Caritamrta-----1:7:128-----purport).

Unintelligent persons who cannot understand this doctrine of byproducts cannot grasp how the cosmic manifestation and the living entity are simultaneously one and different from the Absolute Truth. Not understanding this, one concludes, out of fear, that this cosmic manifestation and the living entity are false. Śaṅkarācārya gives the example of a rope being mistaken for a snake, and sometimes the example of mistaking an oyster shell for gold is cited, but surely such arguments are ways of cheating. As mentioned in the Māṇḍūkya Upaniṣad, the rope for a snake and the oyster for gold examples have their different applications and can be understood as follows. The living entity in his original constitutional position is pure spirit. When a human being identifies himself with the material body, he may be said to be mistaking a rope for a snake, or an oyster shell for gold. The doctrine of transformation is accepted when one thing is mistaken for another.
(Teachings of Lord Caitanya).

Oṁkāra is the reservoir of all the energies of the Supreme Lord. Śaṅkara is wrong in maintaining that the words tat tvam asi are the supreme vibrations of the Vedas, for tat tvam asi are secondary words only. tat tvam asi suggests only a partial representation. In Bhagavad-gītā the Lord has in many places given importance to oṁkāra, (Bg. 8.13, 9.17, 17.24). Similarly, oṁkāra is given importance in the Atharva Veda and the Māṇḍūkya Upaniṣad. In his Bhagavat-sandarbha, Śrīla Jīva Gosvāmī says: "Oṁkāra is the most confidential sound representation of the Supreme Lord." The sound representation or name of the Supreme Lord is as good as the Supreme Lord Himself. By vibrating the sound of oṁkāra, or of Hare Kṛṣṇa, Hare Kṛṣṇa, Kṛṣṇa Kṛṣṇa, Hare Hare/ Hare Rāma, Hare Rāma, Rāma Rāma, Hare Hare, one can be delivered from the contamination of this material world.
(Teachings of Lord Caitanya).

That the sound vibration of the Supreme Lord is identical with the Supreme Lord is a fact. This is confirmed in the Nārada-pañcarātra:
vyaktaṁ hi bhagavān eva
sākṣān-nārāyaṇaḥ svayam
aṣṭākṣara-svarūpena
mukheṣu parivartate
"When the transcendental sound vibration is practiced by a conditioned soul, the Supreme Lord is present on his tongue." In the Māṇḍūkya Upaniṣad it is said that when oṁkāra is chanted, whatever is seen as material is seen perfectly as spiritual. In the spiritual world or in spiritual vision there is nothing but oṁkāra, or the one alternate, om.
(Teachings of Lord Caitanya).

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 byMāṇḍūkya 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.
(Teachings of Lord Caitanya).