Patanjali's yoga sutras

तस्य वाचकः प्रणवः

tasya vācakaḥ praṇavaḥ

The sacred word designating Him is praṇava, the mystic syllable ‘OM’.

तस्य = his (Google translate)

वाचकः = onomatopoeia invocation

प्रणवः = “Praṇava is called this because in a pre-eminent way (pra-) it acknowledges (nava-), i.e. praises i.e. contemplates in its ultimate unity, the nature of the Supreme Lord which encompasses the cosmic totality; as such it is the Indivisible Lord”

My thoughts: If Īśvara is the darkness remover accessible to each one of us, how do we invoke Īśvtara? Through the onomatopoeia sounds that evoke Īśvara’s ultimate unity, the nature of Īśvara which encompasses the Indivisible cosmic totality.

Om is not in the text, but it is yoga’s most well-known Praṇava. My favorite explanation of Om’s origin is that is a combination of Oh, Ah & Mmm, the sounds we make when we understand the mysteries of the universe.