OM is —– The spirit of God, the sound, vibration of this universe that is the absolute truth.
What is Aum then?
AUM is a representation of the three Gods in three planes.
The representation of the three Gods:
A — Stands for Shiva — the past, the birth, rebirth and reincarnation.
U — Stands for Vishnu — the present, the time/space of our universe
M — Stands for Brahma — the future, the manifestation/creation of our universe Om.
The three planes of existence:
Bhur Bhuva Svah
Bhur — Earth —- Lord Shiva
Bhuva — In the sky — Lord Vishnu
Svah — In space — Bharm
Therefore, AUM represents the past, present and future. If you say — the names of Shiva, Vishnu and Bhrama, they do sound true to their representation — past, present and future.