"Om" comes from the first letter in the old Indian alphabet "O" where the mouth is fully open and the last letter of the alphabet "M" where the mouth is fully closed. It means from beginning to end. It also means whatever can be mentioned and explained in words has to lie "within" Om. Literally.
And in the bible it says:
I am the Alpha and the Omega, says the Lord God, "who is & who was & who is to come, the Almighty." -Revelation 1:8