Om is the holy image. It is likewise the essential sound of the Universe. Every single other sound known to man emerges from the sound of Om as it were. It is an enormous sound.
Om is the mix of three letters in particular A, U, and M. An alludes to Anadi (Lord Brahma), U alludes to Upendra (Lord Vishnu), M alludes to Maheshwara (Lord Shiva). In this manner, the word Om (articulated as Aum) likewise means the trinity of Hinduism