I am the father of this universe, the mother, the support and the grandsire. I am the object of knowledge, the purifier and the syllable oṁ. I am also the Ṛg, the Sāma and the Yajur Vedas.
The entire cosmic manifestations, moving and nonmoving, are manifested by different activities of Kṛṣṇa's energy. In the material existence we create different relationships with different living entities who are nothing but Kṛṣṇa's marginal energy; under the creation of prakṛti some of them appear as our father, mother, grandfather, creator, etc., but actually they are parts and parcels of Kṛṣṇa. As such, these living entities who appear to be our father, mother, etc., are nothing but Kṛṣṇa. In this verse the word dhātā means "creator." Not only are our father and mother parts and parcels of Kṛṣṇa, but the creator, grandmother and grandfather, etc., are also Kṛṣṇa. Actually any living entity, being part and parcel of Kṛṣṇa, is Kṛṣṇa. All the Vedas, therefore, aim only toward Kṛṣṇa. Whatever we want to know through the Vedas is but a progressive step toward understanding Kṛṣṇa. That subject matter which helps us purify our constitutional position is especially Kṛṣṇa. Similarly, the living entity who is inquisitive to understand all Vedic principles is also part and parcel of Kṛṣṇa and as such is also Kṛṣṇa. In all the Vedic mantras the word oṁ, called praṇava, is a transcendental sound vibration and is also Kṛṣṇa. And because in all the hymns of the four Vedas-Sāma, Yajur, Ṛg and Atharva-the praṇava, or oṁkāra, is very prominent, it is understood to be Kṛṣṇa.
(Bhagavad-Gita-----9:17-----purport).

Persons who are learned in the Vedas, who utter oṁkāra and who are great sages in the renounced order enter into Brahman. Desiring such perfection, one practices celibacy. I shall now briefly explain to you this process by which one may attain salvation.
Lord Śrī Kṛṣṇa has recommended to Arjuna the practice of ṣaṭ-cakra-yoga, in which one places the air of life between the eyebrows. Taking it for granted that Arjuna might not know how to practice ṣaṭ-cakra-yoga, the Lord explains the process in the following verses. The Lord says that Brahman, although one without a second, has various manifestations and features. Especially for the impersonalists, the akṣara, or oṁkāra—the syllable oṁ—is identical with Brahman. Kṛṣṇa here explains the impersonal Brahman, into which the renounced order of sages enter.
In the Vedic system of knowledge, students, from the very beginning, are taught to vibrate oṁ and learn of the ultimate impersonal Brahman by living with the spiritual master in complete celibacy. In this way they realize two of Brahman's features. This practice is very essential for the student's advancement in spiritual life, but at the moment such brahmacārī (unmarried celibate) life is not at all possible. The social construction of the world has changed so much that there is no possibility of one's practicing celibacy from the beginning of student life. Throughout the world there are many institutions for different departments of knowledge, but there is no recognized institution where students can be educated in the brahmacārī principles. Unless one practices celibacy, advancement in spiritual life is very difficult. Therefore Lord Caitanya has announced, according to the scriptural injunctions for this Age of Kali, that in this age no process of realizing the Supreme is possible except the chanting of the holy names of Lord Kṛṣṇa: Hare Kṛṣṇa, Hare Kṛṣṇa, Kṛṣṇa Kṛṣṇa, Hare Hare/ Hare Rāma, Hare Rāma, Rāma Rāma, Hare Hare.
(Bhagavad-Gita-----8:11-----translation and purport).

My dear King, in a sacred and holy place of pilgrimage one should select a place in which to perform yoga. The place must be level and not too high or low. There one should sit very comfortably, being steady and equipoised, keeping his body straight, and thus begin chanting the Vedic praṇava.
Generally the chanting of oṁ is recommended because in the beginning one cannot understand the Personality of Godhead. As stated in Śrīmad-Bhāgavatam (1.2.11):
vadanti tat tattva-vidas
tattvaṁ yaj jñānam advayam
brahmeti paramātmeti
bhagavān iti śabdyate
"Learned transcendentalists who know the Absolute Truth call this nondual substance Brahman, Paramātmā or Bhagavān." Unless one is fully convinced of the Supreme Personality of Godhead, one has the tendency to become an impersonalist yogī searching for the Supreme Lord within the core of his heart (dhyānāvasthita-tad-gatena manasā paśyanti yaṁ yoginaḥ (S.B.12:13 !). Here the chanting of oṁkāra is recommended because in the beginning of transcendental realization, instead of chanting the Hare Kṛṣṇa mahā-mantra, one may chant oṁkāra (praṇava). There is no difference between the Hare Kṛṣṇa mahā-mantra and oṁkāra because both of them are sound representations of the Supreme Personality of Godhead. praṇavaḥ sarva-vedeṣu. In all Vedic literatures, the sound vibration oṁkāra is the beginning. Oṁ namo bhagavate vāsudevāya. The difference between chanting oṁkāra and chanting the Hare Kṛṣṇa mantra is that the Hare Kṛṣṇa mantra may be chanted without consideration of the place or the sitting arrangements recommended in Bhagavad-gītā (6.11):
śucau deśe pratiṣṭhāpya
sthiram āsanam ātmanaḥ
nāty-ucchritaṁ nātinīcaṁ
cailājina-kuśottaram
"To practice yoga, one should go to a secluded place and should lay kuśa grass on the ground and then cover it with a deerskin and a soft cloth. The seat should neither be too high nor too low and should be situated in a sacred place." The Hare Kṛṣṇa mantra may be chanted by anyone, without consideration of the place or how one sits. Śrī Caitanya Mahāprabhu has openly declared, niyamitaḥ smaraṇe na kālaḥ. In chanting the Hare Kṛṣṇa mahā-mantra there are no particular injunctions regarding one's sitting place. The injunction niyamitaḥ smaraṇe na kālaḥ includes deśa, kāla and pātra—place, time and the individual. Therefore anyone may chant the Hare Kṛṣṇa mantra, without consideration of the time and place. Especially in this age, Kali-yuga, it is very difficult to find a suitable place according to the recommendations of Bhagavad-gītā. The Hare Kṛṣṇa mahā-mantra, however, may be chanted at any place and any time, and this will bring results very quickly. Yet even while chanting the Hare Kṛṣṇa mantra one may observe regulative principles. Thus while sitting and chanting one may keep his body straight, and this will help one in the chanting process; otherwise one may feel sleepy.
(Srimad Bhagavatam-----7:15:31-----translation and purport).

In the Satya-yuga, the first millennium, all the Vedic mantras were included in one mantra-praṇava, the root of all Vedic mantras. In other words, the Atharva Veda alone was the source of all Vedic knowledge. The Supreme Personality of Godhead Nārāyaṇa was the only worshipable Deity; there was no recommendation for worship of the demigods. Fire was one only, and the only order of life in human society was known as haṁsa.
In Satya-yuga there was only one Veda, not four. Later, before the beginning of Kali-yuga, this one Veda, the Atharva Veda (or, some say, the Yajur Veda), was divided into four—Sāma, Yajur, Ṛg and Atharva—for the facility of human society. In Satya-yuga the only mantra was oṁkāra (oṁ tat sat). The same name oṁkāra is manifest in the mantra Hare Kṛṣṇa, Hare Kṛṣṇa, Kṛṣṇa Kṛṣṇa, Hare Hare/ Hare Rāma, Hare Rāma, Rāma Rāma, Hare Hare. Unless one is a brāhmaṇa, one cannot utter oṁkāra and get the desired result. But in Kali-yuga almost everyone is a śūdra, unfit for pronouncing the praṇava, oṁkāra. Therefore the śāstras have recommended the chanting of the Hare Kṛṣṇa mahā-mantra. Oṁkāra is a mantra, or mahā-mantra, and Hare Kṛṣṇa is also a mahā-mantra. The purpose of pronouncing oṁkāra is to address the Supreme Personality of Godhead, Vāsudeva (oṁ namo bhagavate vāsudevāya). And the purpose of chanting the Hare Kṛṣṇa mantra is the same. Hare: "O energy of the Lord!" Kṛṣṇa: "O Lord Kṛṣṇa!" Hare: "O energy of the Lord!" Rāma: "O Supreme Lord, O supreme enjoyer!" The only worshipable Lord is Hari, who is the goal of the Vedas (vedaiś ca sarvair aham eva vedyaḥ (B.G.15:15). By worshiping the demigods, one worships the different parts of the Lord, just as one might water the branches and twigs of a tree. But worshiping Nārāyaṇa, the all-inclusive Supreme Personality of Godhead, is like pouring water on the root of the tree, thus supplying water to the trunk, branches, twigs, leaves and so on. In Satya-yuga people knew how to fulfill the necessities of life simply by worshiping Nārāyaṇa, the Supreme Personality of Godhead. The same purpose can be served in this age of Kali by the chanting of the Hare Kṛṣṇa mantra, as recommended in the Bhāgavatam. Kīrtanād eva kṛṣṇasya mukta-saṅgaḥ paraṁ vrajet (S.B.12:3:51). Simply by chanting the Hare Kṛṣṇa mantra, one becomes free from the bondage of material existence and thus becomes eligible to return home, back to Godhead.

(Srimad Bhagavatam-----9:14:48-----translation and purport).

The impersonal sound of Kṛṣṇa is oṁ, but the sound Hare Kṛṣṇa contains oṁ. The chanting of the Hare Kṛṣṇa mantra is clearly recommended for this age. So if one quits his body at the end of life chanting Hare Kṛṣṇa, Hare Kṛṣṇa, Kṛṣṇa Kṛṣṇa, Hare Hare/ Hare Rāma, Hare Rāma, Rāma Rāma, Hare Hare, he certainly reaches one of the spiritual planets, according to the mode of his practice. The devotees of Kṛṣṇa enter the Kṛṣṇa planet, Goloka Vṛndāvana. For the personalists there are also innumerable other planets, known as Vaikuṇṭha planets, in the spiritual sky, whereas the impersonalists remain in the brahmajyoti.
(Bhagavad-Gita-----8:13-----purport).