"Aum" is a Sanskrit word. Though there are three letters in "Aum", it is nothing but the prolonged sound of 'A' (ah). That means 'A' and "Aum" are identical.
The first vowel 'A' (ah) is not only the first letter of the Sanskrit language but of all languages. This primal sound "A" has started before the creation and has been continuing and will continue in all times to come. All sounds from whichever source they come to us, come out from that one primal source "A" but the sounds differ due to different mediums through which they pass, as we see in the piano, one single sound coming out differently due to its passing through different reeds.
The primal sound can be heard by a perfect Yogi within himself. Everything in this world has a name, and the 'name' and the 'named' (the thing or person possessing the name) are one and the same. Without 'name' you cannot think of anything. This 'name' is wrapped up in that primal sound 'Aum'. So the conclusion comes that everything is pervaded by 'Aum', and the all-pervading thing is nothing but God, so 'Aum' is identical with God. -Hindutvam