By that syllable does the threefold knowledge (the sacrifice, more particularly the Soma-sacrifice, as founded on the three Vedas) proceed. When the Adhvaryu priest gives an order, he says Om. When the Hotri priest recites, he says Om. When the Udgâtri priest sings, he says Om, —all for the glory of that syllable. The threefold knowledge (the sacrifice) proceeds by the greatness of that syllable (the vital breaths), and by its essence (the oblations).

In plain words: All the sacred rituals of the three Vedas, the oldest holy books, move forward through Om. Each priest says Om when he gives an order, recites, or sings, to honor the syllable. The ritual draws its power from Om.