From The Teachings of Lord Caitanya, by Srila Prabhupada:
“Praṇava, or oṁ-kāra, is the chief sound vibration found in the Vedic hymns, and it is considered to be the sound form of the Supreme Lord. From oṁ-kāra all Vedic hymns have emanated, and the world itself has also emanated from this oṁ-kāra sound.”