"OM" is the mystical word which is uttered during Hindu religious practices. This is considered a very holy & auspicious word. What is the meaning of "Om"? How chanting of Om is beneficial to one's body, mind & soul?
In literal sense:
"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."
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.
Jai Sri Krishna