'Om' is the primordial sound of the universe. It is the first sound that was produced at the time of creation, when the 'Hiranyagarbha' the golden womb exploded. This explosion is known as 'Sphota' and through this act, God created the Universe.
Thus, Om is the holiest of all the holy words and mother of all names and forms. Chandogya Upanishad says Om is Bhrahma, the eternal consciousness. Om is compared with Gayatri. It is referred as Tripada Gayatri. The trinity resides in Om. Om represents Brahma, Vishnu and Maheswara.
In Rigveda, Vamadeva Rishi's mantra describes Om as Vishvateeth transcendental. He calls Om as Samudra (sea), Aparimita (infinite), Rooprahita (formless), Sarvavyapi (all pervading). Like the waves hitting the shores are eternal, Om is eternal. In Mundake Uapnishad, Om is referred to as Pranava or eternal. Mundaka speaks about communion of jeevatma with the paramatma. Here Om or Pranava is a dhanush or bow. The atmaa is the sharaa or the arrow. The target is Brahma. You have to shoot the arrow without missing the target i.e. Brahma. This is how jeevatma merges with the paramatma.
In the mandukya upnishad, (the smallest but most important Upnishad), Om is defined as Bramha. Patanjali Yoga sutra calls OM as 'Ishwara pranidhana' i.e. accessing God with devotion. When you meditate on Om , you will be able to arrest the wandering mind and all conflicts will vanish. In the east, the Baudha dharma lays emphasis on Om. In the Namokaar mantra of Jainism, Om occupies a central place as the eraser of all sins.
In Islam, it is referred to as samaa. Samaa is the sound that takes you in communion with Allah. In Sikhkhism, Omkaar is the only truth. Om is auspicious and its chanting helps us in attaining peace. It is a vehicle to take us to the Almighty. All Hindu mantras start with Om.
Thus, OM is the transcendental sound vibration with the potency of liberating the mind from material conditioning.