Yōga Sūtra 1.27

tasya vācakaḥ praṇavaḥ।

HE is conveyed by the pranavah (syllable 'Ōm').

In classical literature out of respect syllable 'Ōm' is never written directly. It is indicated by a name 'Pranava'. The reason is, as explained, it conveys all knowing īśvaraḥ, who is not affected by obstacles, fruits of activities, desires, and who is not bound by time.