As per the Mandukya Upanishad,
"ॐ – Om is the one eternal syllable of which all that exists is but the development. The past, the present, and the future are all included in this one sound, and all that exists beyond the three forms of time is also implied in it".
ॐ is made up of three Sanskrit letters, aa, au and ma which, when combined together, make the sound Aum or Om. It is believed to be the basic sound of the world and to contain all other sounds. Recitation of ॐ give energy to body & Mind. ॐ – Om is a very holy & auspicious word. In literal sense:
"ॐ – Om" is the Sound symbol that represents God. If you want to express God in the form of sound, you should utter this mystical sound "Om". In Bhagwat Gita Lord Sri Krishna says that:
"maharsinam bhrgur aham
giram asmy ekam aksharam
yajnanam japa-yajno ’smi
sthavaranam himalayah" (Bhagwat Gita: Chapter Ten verse 25)
"Sri Krishna said: Of the great sages I am Bhrgu; Of Vibrations / Sounds, I am the transcendental Om. Of sacrifices I am the chanting of the holy names [japa], and of immovable things I am the Himalayas."
pitaham asya jagato
mata dhata pitamahah
vedyam pavitram omkara
rk sama yajur eva cha (Bhagwat Gita: Chapter Nine verse 17)
"Sri Krishna said: I am the father of this universe, the mother, the support and the grandsire. I am the object of knowledge, the purifier and the syllable Om. I am also the Rig, the Sama and the Yajur Vedas."
Before the creation God was one without a second. Then He wanted to become many. At such a thought of God, a vibration came into existence which took the form of sound Om. This vibrational sound Om became the basis of all creation. This primeval sound Om is nothing but another form of God which is recited in religious practices.
When you repeatedly chant this sound Om it gives peace of mind & gives energy to the body. Chanting it elevates the depressed mood of a person. Meditating on it while chanting it lifts our consciousness to the Godly state where peace & happiness comes to our body, mind & soul.
As per "Paramhansa Yogananda":
The outward manifestation of the omnipresent Christ Consciousness (Tat aspect of God), "It's Witness", is Om, the Word or Holy Ghost. This Om is the invisible divine power, the only doer, the sole causative & activating force that upholds all creation through vibration. Om, the Word is the creative vibration that externalizes all creation.
Jai Sri Krishna