Upanishads provide a fine source to learn about Om and its significance. Nothing else could be more authentic and genuine than those in the Upanishads given to us by the Great Seers of the past. The seers are Ṛṣis; they are Brahmanas viz., those who have Realized Brahman, The Ultimate Supreme. The Upanishadic source thus is the most trustworthy source. A good article in the Wikipedia provides information from Upanishads and other sources (4). What I have attempted here is to provide the essence of what is revealed to us about Om in the Upanishads and elaborate some wherever I felt is necessary. 

All major Upanishads point out one essential truth viz., 

• the Sound Om, also referred to as Praṇavam, primordial (primary) and it is the closest to the ultimate Reality viz., Brahman. 
• Om is at the apex of this Vyavaharika (transactional) world. 
• from this Om all Vedas (spiritual scriptures), Divine Forms, human forms and names emerged. 
• meditating on Om is desirable, beneficial and easier to be in tune with the highest level in this transactional world; because Om is you and I. We meditate on ourselves; we simply meditate on who and what we should be; we meditate on our ideal nature. That simple.   

Spiritual upliftment calls for mental purification first. For us, normal humans, worldly life with all its attractive sense-oriented enjoyments have taken over our focus and time. Giving up these acquired enjoyments is a huge challenge and task.  For us with all our bodily ailments, even giving up or reducing food habits to control diabetics or blood pressure is a Herculean task. The competition is always between worldly (more precisely bodily) and Godly.  Usually worldly wins because we have not experienced Godliness and we have experienced our bodies. We assume that there is a God from our childhood because we are told by elders and ancient scriptures. Unless we try and work to uncover Divinity in us, we would not have the strong belief that God as a Super Being exists. Our thoughts, then, would remain as someone’s words with no real hold in us to strive to uncover that Truth. It is just like someone told that sugar is sweet and we find it to be true only when we taste sugar. In fact, we correlate the word sugar with that sweet taste only after we tasted it; the correspondence between that word sugar and the taste sets in only after the experience. Is it not? We need to similarly attempt to correlate the word Om with Divinity. Swami’s miracles are only for this purpose viz., to make us aware that Divinity exists and to strive for that. 

Humans are always engaged in action for one reason or another. Adi Sankara has thus clearly stated in His introduction to Chandogya Upanishad Bhaṣyam (explanatory notes), that it is for this reason that the Upanishad advocates meditation on Om pertaining to the actions in which humans are involved. It makes it easier for the mind to embrace meditation. Therefore, rituals, as prescribed in the Vedas along with the worship of the Divine Forms, becomes an easier stepping stone in the climb towards higher aspects of Divinity and Brahmam.  Vedic rituals and pooja practices thus are critical steps to go up the ladder.  Rituals conducted as per Vedic injunctions engage one for a transition from worldly to Godly activities. 

Chandogya Upanishad is considered as the “s’iras” (head) of all Knowledge. That is why, right at the beginning of Bhaṣyam for the Upanishad, Adi Sankaracharya, the most revered champion of Non-Duality principle, declares that Om iti ētat aksharam (this Om is one syllable) (5 ** Chandogya Upanishad, Introduction by Adi Sankaracharya. Dr. Sir Gananatha Jha, Oriental Book Agency, Poona 1942. ). This one syllable, Om, is all embracing and an assent for all rituals and activities.  The same concept that Om is One Syllable (Om iti ētat aksharam ) that embraces everything in this Maya world is stressed at the very beginning in Māndūkya Upanishad too (6 ** Māndūkya Upanishad, 1).

This syllable Om, this sound, this S’abda Brahman, is considered the First Expression of Divinity that was dormant and unmanifested Energy. Adi S’ankara in a stuti (hymn, prayer) to S’akti Meenakshi starts by singing “S’abda Brahma Mayi “ (meaning: all pervading Brahma Sound). 

Om and its effect are thus necessary for all spiritual activities and rituals; it is very well enunciated in Taittreya Upanishad (7 **Taitreya Upanishad,  S’iksha Valli, Eighth Anuvaka.).  It declares that Brahma assents by the word Om to Adhvaryu, Hotru and Udgātṛu (they are Brahmanas- Knowers of Brahman) and they all invoke Om in each of their acts in rituals. Taitreyam elaborates that Om means Brahman;  Om means all this;  Om means obedience. When they have been told, ‘Om, speak,’ they speak;  after Om they sing Sāmans; after Om they recite hymns; after Om the Adhvaryu gives the response;  after Om the Brahman-priest gives orders;  after Om he (the sacrificer) allows the performance of the Agnihotra; when a Brahmana is going to begin his lecture, he says,  ‘Om, may I acquire Brahman (the Veda).’ He thus acquires the Veda (7). 

Thus, for every act in prayer or sacrifice or any ritual, assent is obtained by chanting Om by a Brahmana. This chant of Om by the Realized Brahmana at the rituals invokes the S’akti within Himself to Bless and conduct the rituals. Therein lies the importance of Om implying that nothing can happen without Om. From Chandogya Upanishad we understand that any mantra not starting with Om is ineffective (5). Bhagawad Gīta also says that all yagnas, gifts and penance activities are done with the pronunciation of the name of Supreme Atma Om (8 **Bhagawad Gīta, Chapter 17, 24).  

How did this syllable come? Chandogya reveals that “Prajapati (Creator) brooded over and there issued the syllable Om; just as all leaves are permeated by the stalk, so is all speech permeated by the syllable Om. The Om syllable is all this” (9 **Chandogya Upanishad, 2-23-3.). Sankara says that the entire Vedas are but OM and all issues out of it and consists of It. Sankara says Om within itself contains the entire literature (all knowledge).  

What should we do with that? Chandogya declares that one should meditate on the syllable Om and the syllable OM is the most appropriate name of the Supreme Self (5). According to Sankara Om is the most appropriate Form of Brahman; Om is thus the best Name and Form; When that syllable is pronounced the Supreme Self is pleased, just like a person is pleased by the name he likes best says Sankara in His Bhaṣyam of Chandogya; the name and sound Om is the best representation of Brahman. And therefore, Om serves as means to meditate on Brahman. Not only that, Pras’na Upanishad takes an extra step and mentions that one should meditate on the Supreme Self ONLY through Om (OmkaareNe Aayatanena anveti) (10 **Pras’no Upanishad, 5: 7).