So, here is the whole cycle of creation, preservation and destruction. God–Generating, operating and destroying if you were to think that way! The cycle is Om, symbolised by Om.
Now, another scripture, an Upanishad that is part of the Vedas–the Mundaka Upanishad–the shortest of the Upanishads, also gives definition of Om, which is a little different.
It says that ‘aa‘ stands for Jagratha Awastha, which means the waking state of consciousness, which is common to all human beings. ‘Oo‘ represents the dream state which is also called Tejasa–Swapna Awastha that is again common to all human beings. And, then ‘m‘ or ‘im‘ represents Sushupti or deep sleep that is again common to all beings.
The Ardh matra accompanying Om, which is the vibration of the sound that is similar to the sound that slowly begins to disappear after you ring a bell. The ringing sound takes quite sometime to disappear. That is called the Ardh matra –that which is a witness of all the three states–waking, dream and deep sleep. The eternal witness that is our true self, which watches the drama and yet is not affected by it.
Therefore, the Om represents the whole creation, the whole universe, the cosmos, the human being, the connection between a human and the universe, the connection between the individual and the Supreme all-pervading being. This is what Om is.