Answer:
OM as us hindus believe is a universal sound. When the world was being created by lord vishnu, lord Shiv and lord Bhrama then all you could hear was Ommmmm. Om was then used when you pray to represent the unity or strength of the sound. Your body is made of chakras and this sound aligns the chakras which helps find you innder peace and self, which is why you say om while meditating or praying...
hope this answers you question
nfjn..