Om is a basic symbol of Hinduism. Often it is also called 'Aum'. But the precise transcription does not matter for the important part is the sound: a long stretched vowel that floats between the 'ooh' and 'auw' and that ends with a murmuring 'mmmm...'. And that sound is expressed with a specific symbol: a round 'three' with an elegant curl in the middle and, on the top, a moon shape lying down with a determined dot within. The sign is omnipresent in India, is visible on many amulets you can buy on festivals and is a recurrent theme in stores with exotic material for interior decoration. Yet no matter how present it might be in spiritual and so called 'alternative' circles, often it isn't comprehended correctly.
Om signifies the basic vibration of the universe. It represents the essential rhythm of energy of which all matter is build up. It stands for the primordial sound that pervades everything and therefore brings about the vibration that keeps the whole of the universe in motion.
On an atomic level it is also rather undeniable that the smallest particles are in constant motion. Even if matter often seems motionless for those who only look with their eyes, its tiniest building blocks are constantly whirling around each other just like the planets around the sun.
When one takes the Om sound as a mantra or when one inwardly meditates upon it, one aims for unification with that basic vibration of the universe. Meditation with a focus on Om is therefore intended to make one perceive the basic rhythm of the world and to let one's very own being be fulfilled by that vibration.
And indeed, those who do so correctly will quickly become aware of a deep and intense energetic vibration. One can thus truly open oneself to Om and feel it within. Even more so, one can try to accord one's 'movements' – that is to say one's actions – with it.
In some spiritual circles however, they even go a step further. Some see the taking up the vibration of Om as a way of making direct contact with God. They then let it seem like Om is the divine vibration that created the whole of the universe. And from this often follows that one who is one with the frequency of Om, is one with God.
Yet this isn't what Hinduism truly says. And spiritually speaking it makes no sense. The vibration, the basic energy isn't God. God is present within that vibration and energy, but they are not the same. God is much larger than that energy. God goes much deeper than that vibration.
God simply hides within everything. And therefore also in the vibration and energy of Om. So even though a person can truly come to certain spiritual insights and can experience a spiritual reality when he discovers the basic tone of Om, by nearing Om he has not discovered the total reality of the divine.
The energy of Om is thus a fundamental part of existence, but not the start and end. If Om is the rhythm of the universe, God is the heartbeat of that rhythm. If Om is a basic chord, God is the melody.