Omkar is a religious symbol of the Hindus. But it is omnipresent in almost all the major cultures of our world.
Another way of saying Om is Aum. Aum is the sound of the infinite. Aum is said to be the essence of all mantras and Vedas, the highest of all mantras or divine word. By sound and form, AUM symbolizes the infinite Brahman and the entire universe.
A stands for Creation.
U stands for Preservation.
And finally M stands for Destruction or dissolution.
This represents the Trinity of God in Hindu dharma (Brahma, Vishnu and Shiva).
The three portions of AUM relate to the states of waking, dream and deep sleep and the three guans (rajas, satva, tamas). The three letters also indicate three planes of existence, heaven, earth and the netherworld. AUM can represent all the words produced by the human vocal organ. A is produced by the throat, U & M by the lips. In the Vedas, AUM is the sound of the Sun, the sound of Light. It is the sound of assent. It has an upward movement and uplifts the soul, as the sound of the divine eagle or falcon.
The pronunciation of the word "OM" is supposed to symbolize the totality of all sounds as it includes all other sounds that humans can utter. This idea of totality also exists in the English word "Omnipresent" that includes OM as its prefix. We also have words like Omnipotent and Omniscient, all of which have the concept of totality in their meanings.
The ancient Greek alphabet had Omega as its last letter. Omega written in the lower case of the Greek alphabet, if turned to its side, looks quite similar to the Sanskrit way of writing Om. It is from the Greek alphabet "Omega" that we have the English phrase "the alpha and Omega", which means, "to include everything". It is said that the word Om has been used to make other words. The Christian term "Amen" is said to have some link with "Om" as also the Islamic term "Amin". Both of these terms are similar to Om.
Om is a well-known symbol to Indians all over the world. Almost every Indian household owns some sort of Om symbol. The way I was taught Om is a symbol of God.