Om (also written Aum) is the seed-syllable of the Cosmic Spirit. Mahendranath wrote that as fire is known by its heat, Om is known by its vibration.
Within the International Nath Order it opens rite and mantra: Om Namah Shivaya, Om Gam Ganapati Namah, and the bow to the Cosmic Spirit in The Puja of the Three Super Faculties. It is a sound to be used, not a creed to be believed. It is not a substitute for japa of a given formula, and it is not a grade. This page names the syllable.