Om is the essence of everything (Taittiriya Upanisad 1.8)
An impersonalist may chant Om to address the impersonal Brahman, a yogi may pronounce om referring to Paramātmā inside his heart, while a devotee understands that om is the Lord in His personal form.
Section 8: Om is the essence of everything
The syllable om is very central in Vedic culture. Om is the holy name that makes the Lord present. An impersonalist may chant om to address the impersonal Brahman, a yogi may pronounce om referring to Paramātmā inside his heart, while a devotee understands that om is the Lord in His personal form. The syllable om represents the Lord in all His aspects, one is just limited by his own comprehension.
Text 1.8.1
om iti brahma om itīdam sarvam
om ityetad anukṛtir ha sma vā apyo śrāvayety āśrāvayanti
om iti sāmāni gāyanti , om śam iti śastrāṇi śamsanti
om ity adhvaryuḥ pratigaram pratigṛṇāti , om iti brahmā prasauti
om ity agni-hotram anujānāti om iti brāhmaṇaḥ pravakṣyann āha brahmopāpnavānīti brahmaivopāpnoti
Om is indeed Brahman, the ultimate reality. Om is the essence of everything. Om is chanted to consent to all duties and prohibitions. Chanting om śravāya, they begin to recite. The udgātṛs sing sāma verses chanting om. The hotṛs chant the mantras, starting with om śam. The adhvaryu chants om in response to the hotṛ. The brahmā priest chants om to give permission. The sponsor gives permission for the sacrifice by chanting om. The brāhmaṇa chants om before studying the Vedas, saying “Let me study the Vedas.”
Commentary: The syllable om is very central in Vedic culture. Om is the holy name that makes the Lord present. The syllable om is used in all steps of a fire sacrifice, is pronounced before one studies the sastras, and is even used to enter into a contract. Since om invokes the presence of the Lord, if one agrees to do or not do something by uttering om, he cannot take his word back after that. Om can be used to celebrate any kind of agreement, to make promises, or even to celebrate a marriage.
An impersonalist who sees the Supreme Lord as an impersonal entity may chant the syllable om to avoid personal names of the Lord, such as Rama or Krsna, but in any case, there is no difference. The syllable om represents the Lord in all His aspects, one is just limited by his own comprehension. In the Srimad Bhagavatam 1.2.11 it's explained:
vadanti tat tattva-vidas, tattvam yaj jñānam advayam
brahmeti paramātmeti, bhagavān iti śabdyate"Learned transcendentalists who know the Absolute Truth call this nondual substance Brahman, Paramātmā or Bhagavān."
An impersonalist may chant om to address the impersonal Brahman, a yogi may pronounce om referring to Paramātmā inside his heart, while a devotee understands that om is the Lord in His personal form. Om is, however, easier for impersonalists and others who are not ready to accept names that directly describe the form and pastimes of the Lord.
The contents of this verse are explained in much more detail in Prabhupada's purport to Caitanya Caritāmṛta Adi 7.128:
"Omkāra is the basic principle of all Vedic mantras, for it is a representation of Lord Kṛṣṇa, understanding of whom is the ultimate goal of the Vedas, as stated in the Bhagavad-gītā (vedaiś ca sarvair aham eva vedyaḥ (Bg 15.15)).”
“As mentioned above, the transcendental vibration omkāra is explained in the Bhagavad-gītā, Chapter Eight, verse thirteen:
om ity ekākṣaram brahma vyāharan mām anusmaran
yaḥ prayāti tyajan deham sa yāti paramām gatim
"After being situated in this yoga practice and vibrating the sacred syllable om, the supreme combination of letters, if one thinks of the Supreme Personality of Godhead and quits his body, he will certainly reach the spiritual planets." If one actually understands that omkāra is the sound representation of the Supreme Personality of Godhead, whether he chants omkāra or the Hare Kṛṣṇa mantra, the result is certainly the same.
The transcendental vibration of omkāra is further explained in the Bhagavad-gītā, Chapter Nine, verse seventeen:
pitāham asya jagato mātā dhātā pitāmahaḥ
vedyam pavitram omkāra ṛk sāma yajur eva ca
"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 om. I am also the Ṛg, the Sāma and the Yajur Vedas."
Similarly, the transcendental sound om is further explained in the Bhagavad-gītā, Chapter Seventeen, verse twenty-three:
om tat sad iti nirdeśo brahmaṇas tri-vidhaḥ smṛtaḥ
brāhmaṇās tena vedāś ca yajñāś ca vihitāḥ purā
"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 brāhmaṇas while chanting Vedic hymns and during sacrifices for the satisfaction of the Supreme."
Throughout all the Vedic literatures the glories of omkāra are specifically mentioned. Śrīla Jīva Gosvāmī, in his thesis Bhagavat-sandarbha, says that in the Vedic literature omkāra 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 māyā. Sometimes omkāra is also called the deliverer (tāra). Śrīmad-Bhāgavatam begins with the omkāra vibration: om namo bhagavate vāsudevāya. Therefore omkāra has been described by the great commentator Śrīdhara Svāmī as tārānkura, the seed of deliverance from the material world. Since the Supreme Godhead is absolute, His holy name and His sound vibration omkāra are as good as He Himself. Caitanya Mahāprabhu says that the holy name, or omkāra, the transcendental representation of the Supreme Personality of Godhead, has all the potencies of the Personality of Godhead.
nāmnām akāri bahudhā nija-sarva-śaktis
tatrārpitā niyamitaḥ smaraṇe na kālaḥ
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 omkāra, is the Supreme Personality of Godhead Himself. In other words, anyone who chants omkāra 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, om 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 omkāra.
On the basis of all the Upaniṣads, Śrīla Jīva Gosvāmī says that omkāra is the Supreme Absolute Truth and is accepted as such by all the ācāryas and authorities. Omkāra is beginningless, changeless, supreme and free from deterioration and external contamination. Omkāra is the origin, middle and end of everything, and any living entity who thus understands omkāra attains the perfection of spiritual identity in omkāra. Omkāra, being situated in everyone's heart, is īśvara, the Supreme Personality of Godhead, as confirmed in the Bhagavad-gītā (18.61): īśvaraḥ sarva-bhūtānām hṛd-deśe ’rjuna tiṣṭhati. Omkāra is as good as Viṣṇu because omkāra is as all-pervasive as Viṣṇu. One who knows omkāra and Lord Viṣṇu to be identical no longer has to lament or hanker. One who chants omkāra no longer remains a śūdra but immediately comes to the position of a brāhmaṇa. Simply by chanting omkāra one can understand the whole creation to be one unit, or an expansion of the energy of the Supreme Lord: idam hi viśvam bhagavān ivetaro yato jagat-sthāna-nirodha-sambhavāḥ. "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." (SB 1.5.20) Although one who does not understand concludes otherwise, Śrīmad-Bhāgavatam 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, omkāra."
You can find even more details in the purport, this is just the most relevant part.