In the Upaniṣads, om is an all-encompassing sound, essentially an emanation of Brahman. Since Brahman is in everything, so is om.
According to the Upaniṣads, divine consciousness first manifested as the sound om. Before that, there was only void, or “śūnyākāśa” (“empty sky”).
So everything in the world ultimately started with om. Thus, when a sacred scripture is recited, one first intones “om” as the start of the consciousness and words of God.