From the beginning of creation, the three syllables om tat sat have been used to indicate the Supreme Absolute Truth [Brahman]. They were uttered by brahmanas while chanting Vedic hymns and during sacrifices for the satisfaction of the Supreme."
Throughout all the Vedic literatures the glories of omkara are specifically mentioned. Srila Jiva Gosvami, in his thesis Bhagavat-sandarbha, says that in the Vedic literature omkara is considered to be the sound vibration of the holy name of the Supreme Personality of Godhead. Only this vibration of transcendental sound can deliver a conditioned soul from the clutches of maya. Sometimes omkara is also called the deliverer (tara). Srimad-Bhagavatam begins with the omkara vibration: om namo bhagavate vasudevaya. Therefore omkara has been described by the great commentator Sridhara Svami as tarankura, the seed of deliverance from the material world. Since the Supreme Godhead is absolute, His holy name and His sound vibration omkara are as good as He Himself. Caitanya Mahaprabhu says that the holy name, or omkara, the transcendental representation of the Supreme Personality of Godhead, has all the potencies of the Personality of Godhead.
namnam akari bahudha nija-sarva-saktis
tatrarpita niyamitah smarane na kalah
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 omkara, is the Supreme Personality of Godhead Himself. In other words, anyone who chants omkara and the holy name of the Lord, Hare Krishna, immediately meets the Supreme Lord directly in His sound form. In the Narada-pancaratra it is clearly said that the Supreme Personality of Godhead Narayana personally appears before the chanter who engages in chanting the ashtakshara, or eight-syllable mantra, om namo narayanaya. A similar statement in the Mandukya Upanishad declares that whatever one sees in the spiritual world is all an expansion of the spiritual potency of omkara.
On the basis of all the Upanishads, Srila Jiva Gosvami says that omkara is the Supreme Absolute Truth and is accepted as such by all the acaryas and authorities. Omkara is beginningless, changeless, supreme and free from deterioration and external contamination. Omkara is the origin, middle and end of everything, and any living entity who thus understands omkara attains the perfection of spiritual identity in omkara. Omkara, being situated in everyone's heart, is isvara, the Supreme Personality of Godhead, as confirmed in the Bhagavad-gita (18.61): isvarah sarva-bhutanam hrid-dese 'rjuna tishthati. Omkara is as good as Vishnu because omkara is as all-pervasive as Vishnu. One who knows omkara and Lord Vishnu to be identical no longer has to lament or hanker. One who chants omkara no longer remains a sudra but immediately comes to the position of a brahmana. Simply by chanting omkara one can understand the whole creation to be one unit, or an expansion of the energy of the Supreme Lord: idam hi visvam bhagavan ivetaro yato jagat-sthana-nirodha-sambhavah. "The Supreme Lord Personality of Godhead is Himself this cosmos, and still He is aloof from it. From Him only this cosmic manifestation has emanated, in Him it rests, and unto Him it enters after annihilation." (Bhag. 1.5.20) Although one who does not understand concludes otherwise, Srimad-Bhagavatam states that the entire cosmic manifestation is but an expansion of the energy of the Supreme Lord. Realization of this is possible simply by chanting the holy name of the Lord, omkara.