Significance of Om :
Similar argument they use to give it for other mantras that were in Sanskrit, but I was never satisfied with these answers as I use to argue that after all they are just words, and what will be the effect of it if I don’t know its meaning for eg: if I say that ” I am a Dog” in sanskrit how the hell will it create positive vibrations in me.
Later on with time I realised that OM is the sound produced in the process of completion of a complete cycle of breath. If you observe closely, the moment you inhale your breath sound “O” is produced and when you exhale sound “M” is produced.
It is the only word in this world which can be produced inside your mind without disturbing the entire process of breathing. It will become more clear when your breath will become “Pran” i.e. the air that you are inhaling while reading this para.
When you will be able to concentrate on your breath i.e. “pran” without putting any effort in pulling your breath and your exhale will become “Aapan” i.e. the air that you are exhaling at the moment without pushing you breath.
OM is of great help while mastering the process of “Pranayam” i.e. mastering your concentration over your “Pran” & “Apan”, try out “RAM” or any other word in your mind while mastering the act of “Pranayam”, the flow of breath will be missing.
Pranayam is a creation of very advance thinking, what we are doing today is just maintaining the legacy of our ancestor in a pretty distorted form.