ऋचो अक्षरे परमे व्योमन यस्मिन देवा अधि विश्वे निषेदुः |
यस्तन्न वेद किमृचा करिष्यति य इत्तद्विदुस्त इमे समासते || RV .1.164.39 ||
This Rishi of this Mantra is Dirghatamas and is Known as "Asya Vamiya Sukta " . The Mantras 1-41 are dedicated to Vishwadeva.
In this Mantra Dirghatamas is pointing towards those who do not know the Imperishable Brahman....
Yes. Om is mentioned in the Veda Samhitas. It first occurs in Yajur Veda.
The butter's rapid flow delight his spirit! Brihaspati extend this act
of worship. May he restore the sacrifice uninjured. Here let all Gods
rejoice. OM! Step thou forward. (Shukla Yajur Veda 2.13).
Pranava is also mentioned in Isha Upanishad, i.e, Shukla Yajur Veda 40: