Khandogya Upanishad

Meditate on the syllable Om. This syllable is sometimes called the Udgitha because a portion of the Samaveda (the Udgitha) is sung beginning with Om. The nature of all beings of the earth, all material and matter, animate and inanimate, all speech, the essence of the Rigveda and Samaveda can be understood by meditating on this one syllable: by understanding that speech coarises with breath, you can understand the codependent nature of suffering. The meditation of Om is one of permission, of permitting things to coexist, coarise, coterminate: just as when two people come together in friendship and love, they would naturally fulfil each other's desire, so does breath and speech fulfill the other's purpose - and by understanding this, you can fulfill your purpose. By such meditation we understand the nature of sacrifice. The Priest gives an order and says "Om!" When the Hotri Priest (the one charged with invocation and litany of hymns) recites, they say "Om!" When the Udgartri Priest (the one charged with singing the praises of the sacrifice) sings, they say "Om!" Om! Om! Om! All because of the importance of meditating on that one syllable. Whoever knows the meaning of Om, having meditated on it and understood it, and whoever does not understand Om - both would perform the same sacrifice. But whoever does not understand Om would lack the knowledge and faith the sacrifice would bring - and give. This is why it is said there is a secret meaning of Om. - Khandogya Upanishad 1.1