om refers to Atman (soul, self within) and Brahman (ultimate reality, entirety of the universe, truth, divine, supreme spirit, cosmic principles, knowledge
The mantra “so hum” is not only a reflection of the sound of the breath but also carries a contemplative meaning: “I am that” (so = “I am” and hum = “that”). Here, “that” refers to all of creation, including the breather, the one breathing us all, us all breathing as one. it means identifying oneself with the universe or ultimate reality
And then there is the mantra of this guy
Om So Long