This wiki article says that all Vedic rituals begin with the chanting of OM.
But it's actually more than that. Not only all Vedic recitations must begin with the Pranava (OM) but it should also end with its recitation.
Now, this concept of sanctifying the main mantra body, by applying OM both at the start and the end of the body, is widely known in the Agamas or the Tantras.
And, for Vedic mantras too, i found a similar evidence from the Manu Smriti:
Manu Smriti 2.74. Let him always pronounce the syllable Om at the
beginning and at the end of (a lesson in) the Veda; (for)
unless the syllable Om precede (the lesson) will slip away (from him), and unless it follow it will fade away.
So, not only all Vedic recitals must begin with the chanting of OM but they should also end with its chanting.