ॐ, oṃ, auṃ
ओंकार, oṃkāra [the syllable oṃ]

The primary sound; the highest mantra, pranava. 

OM is the first manifestation of the impersonal Brahman in the form of sound (Shabda-Brahman), the symbol of Absolute. It is believed that the Universe came from the vibration caused by this sound. The three letters AUM symbolize the divine trinity: Brahma (aspect of creation), Vishnu (aspect of preservation) and Shiva (destruction aspect), as well as the three gunas (rajas, sattva and tamas), of which, in combination with 5 elements, the world is created. Ardhachandra (curved line in the form of a crescent) denotes Shakti or Maya, through which the gunas are manifested, and the point is Bindu, the cause of all creation.

Shivaism says that Omkara is generated by the sound of Shiva's drum – Damaru. Buddhism connects OM with bodies of Buddha – Dharmakaya, Sambhogakaya, Nirmanakaya.

In the Nath Tradition, the original sound of OM is of paramount importance, it is believed that the first Guru of Nathas, Shiva Adinath, is Omkar-svarupa (form of the sound OM). In yoga practice, pranava OM is used in nadanusandhana (in fact, the transcendental sound of Nada is OM vibration), in pranayama, and also in dvadashanta

In the attributes of Natha-yogis, OM is symbolically represented in nadi-janeu: the black thread symbolizes emptiness, the pavitra (ring) – Maya, the sphatik – Vishnu, rudraksha – Maheshvara, the coral – Brahma, and the nadi whistle – bindu (as the quintessence of Nada sound). 

A lot of shlokas in the most diverse spiritual writings describe Omkara, its meanings and practices associated with it:

Aum! This sound is all. Here is its explanation: past, present, future – all this is the sound of Aum. And that other, that outside of time is sound Aum too. This is the universal lord, this all-knowing, this is the inner ruler, this is the source of everything, the beginning and the end of the beings. This is Atman in relation to sounds, the sound of Aum in relation to parts. The feet (of Atman) are the parts, and the parts are the feet: the sound a, the sound u, the sound m

Mandukya Upanishada

Three parts of time, three worlds, three svaras, three deities reside in them. This is Om, the Supreme Light. (89)
Sattva, rajas and tamas, Brahma, Vishnu and Mahesvara – these three gods reside in the three matras of Om. This is Om, the Supreme Light. (90)
Kriya, iccha and jnana, Brahmi, Raudri and Vaishnavi – this triple shakti resides in the three matras of Om. This is Om, the Supreme Light. (91)
If yogi constantly practices the pranava of Om, whether he is pure or unclean, he gets no sin, as a lotus leaf does not get wet from water. (92) 


oṃ khaṃ brahma | khaṃ purāṇam; vāyuraṃ kham iti ha smāha kauravyāyaṇīputraḥ; vedo'yaṃ brāhmaṇā viduḥ; vedainena yadveditavyam || 1 ||

"Om is the ether (space), Brahman, the eternal ether. ‘The ether containing air,’ says the son of Kauravyāyaṇī. It is the Veda, (so) the Brāhmaṇas (knowers of Brahman) know; (for) through it one knows what is to be known."

Brihadaranyaka Upanishada (5.1)