Om

OM
The most sacred syllable in Hinduism is OM.
It stands for Brahman, both as personal and impersonal God. The passage below is one of the clearest of the countless references to OM in the Hindu scripture.

The Meaning of Om

What was the precise meaning of the word 'Om' is not known! It is certainly related to the God/Heaven, but in what way is yet to be found out!

Gayatri Mantra (गायत्री मंत्र; gaayatree maNtra) like many other vedik shlokas starts with Om. That's why I assumed that Om (ओम) means 'My Lord' or 'Oh, our God' ('हे परमेश्वर').

Purānic Hinduism

In Purānic Hinduism, Aum is the mystic name for the Hindu Trimurti, and represents the union of the three gods, viz. a for Brahma, u for Vishnu and m for Mahadev which is another name of Shiva. The three sounds also symbolize the three Vedas (Rigveda, Samaveda, Yajurveda).

According to Hindu philosophy (see Māndukya Upanishad), the letter A represents creation, when all existence issued forth from Brahma's golden nucleus; the letter U refers to Vishnu the God of the middle who preserves this world by balancing Brahma on a lotus above himself, and the letter M symbolizes the final part of the cycle of existence, when Vishnu falls asleep and Shiva has to breathe in so that all existing things have to disintegrate and are reduced to their essence to him. More broadly, Aum is said to be the primordial sound that was present at the creation of the universe. It is said to be the original sound that contains all other sounds, all words, all languages and all mantras.

The Om symbol

An 'Om symbol' is a sign (glyph, shape) to write (scribe, represent) the 'Om' sound.
'Om symbol' is called 'Omkar' (ओम्-कार; Omkaar) or 'Onkar' (ओंकार; ONkaar). ओंकार can be written as ओँकार (ONnkaar).
As discussed earlier, the 'Om' sound can be written as 'ओम' in DevaNagari. So 'ॐ' is a kind of shortcut of the word 'ओम' in DevaNagari and other Indic scripts.
People use this sign/symbol for meditation and worship.
Om symbol is drawn mostly as in the picture below or as 'ॐ'.

Onkar in Sikhism

Ik Onkar (One God)

Ik Onkar ੴ in Gurmukhi script, a combination of the numeral one ੧ and the letter ੳ with the vowel marker hōṛā, from Sanskrit ekomkāra "one omkāra,") is a symbol of the unity of God in Sikhism, Guru Nanak having used the Sanskrit-based term to portray the unique theological conception of One God. The Ik Onkar ligature is encoded as a single character in Unicode at U+0A74 (ੴ). The term onkar is found in Sikhism, expressed as Ik Onkar (One God), and emphasizes monotheism, in contrast to the multiplicity of "om" The Sikh concept of God (Waheguru) is summarized by the first composition of Guru Nanak, the Mul Mantar. In the Mul Mantar, Guru Nanak writes "One God, the true name, the creator, without fear, without hatred, timeless, self-existent" The singularity of God is expressed in the saying, Ik Onkar ("There is one God").

(To Mahima) "You explain 'Aum' with reference to 'a', 'u', and 'm' only."
Mahima: "'A', 'u', and 'm' mean creation, preservation, and destruction."
Master: "But I give the illustration of the sound of a gong: 'tom', t-o-m. It is the merging of the Lila in the Nitya: the gross, the subtle, and the causal merge in the Great Cause; waking, dream, and deep sleep merge in Turiya. The striking of the gong is like the falling of a heavy weight into a big ocean. Waves begin to rise: the Relative rises from the Absolute; the causal, subtle, and gross bodies appear out of the Great Cause; from Turiya emerge the states of deep sleep, dream, and waking. These waves arising from the Great Ocean merge again in the Great Ocean. From the Absolute to the Relative, and from the Relative to the Absolute. Therefore I give the illustration of the gong's sound, 'tom'. I have clearly perceived all these things. It has been revealed to me that there exists an Ocean of Consciousness without limit. From It come all things of the relative plane, and in It they merge again."

-- The Gospel of Sri Ramakrishna ²